Always Henesy - does it take a month to sort out, or does it depend on the issue? Would you mind giving me an idea of how much it costs (PM me if preferred). I'd be grateful to have an idea. Presumably something like this you can't sort out by attending his clinics?
By the way - I love your springer, I've got one just like that. They're gorgeous
He did not give you warning, as such, but felt like any other fresh horse. Sometimes he would stick his tail up before he went - he galloped like that, too - but usually it just happened. It was not "fixable", we just had to manage it. He got better as he got older but the first few hacks in spring stayed exciting until he was in his dotage. He was like Spookypony's horse, in that people brushed it off until they saw him do it! I found out later everyone thought I was mad for riding him (it wasn't his only party trick) but it must have looked worse than it felt. Get him assessed by someone with this area of speciality, get their advice on how to sort it and give it a bit of time.
Spinning is "just" a behaviour - like bucking or rearing or snorting, for that matter - which a horse uses to release tension. So there isn't a "one size fits all" cure, it is always a matter of learning how to deal with/channel the behaviour and also finding ways to help the horse dial down the tension that results in it in the first place.
I had one horse that "spun for fun". He did it most often in situations where he was fresh or excited, or if he was anxious about something. He could and would spin both ways and at "nothing" - we rode in doors all winter and by February I'd be lucky to make it once around the school without a spin until he'd worked for a bit. He was also not above adding a huge buck on the end of it, which was quite effective.
My very experienced coach set out to "fix" him one day on a hack and it did not go well . . .the horse must have spun about a hundred times down the lane from the farm! I know that someone previously had tried to fix it with the stick and that had, in fact, made it worse, so in that case his tension was ramped up by the attitude and every time he was "disciplined" for it, he did it again. It was a bit grim but kind of funny and when my coach returned he literally never mentioned it again.
That horse was very unusual, though. Most spinners I've met do it out of anxiety and usually in concert with an issue in their way of going that might not seem like much of a problem the rest of the time.
One I had did it because he hurt. He was far more likely to do it when asked to work a particular way and in his case, sorting out his physical issues helped a lot. He was also a very nervy, sharp horse in general, so management, again, played a big role, especially attention to his diet and his care routine. Having learned from that one, with similar mare I had to train, we did a bute trial on her and the spinning stopped, so the owner knew to look further. In these cases though, the spinning was varied and it wasn't really "out of the blue" just very quick and preceded only by a moment of "ouch" tension.
Far more common is horses that have a perfectly understandable anxiety - collecting rings, say, or hacking alone - and is predisposed to spin, because of way of going or physical factors, or the way it's being ridden. The spin is effect the "open door" and when the tension gets too much to bear, the horse takes it. This sounds more like your horse, OP.
One I had to ride recently was far more typical than the one I mentioned above. He generally went a bit "over his shoulder" on the flat, with a kink and a tenancy to take one rein more strongly, although he was going out and doing tests and doing well. He originally hacked fine but then, after a "bad" experience (it wasn't particularly dramatic, but horses don't always see things the same way we do when it comes to anxiety, what with being overgrown rabbits and all), started to spin randomly out hacking and occasionally in the school. He was awfully quick with it! Unfortunately the rider did the "instructed" thing and shortened her reins but also shortened his neck, making him more uncomfortable and exacerbating the "kink" in his body that made a spin (to the left in his case) seem like the default way to go. The "fix" was two fold, to improve his way of going, most notably in straightness and stretch to the hand, and then to ride him in this more "relaxed but controlled" way in the stressful situations to both give him confidence, and if needed, discipline him sooner, when he started his tiny lean, rather than waiting until he's already spun. The owner continued to ride him in the school while I worked with him, then we did a gradual switch over, although I still ride him fairly regularly and will likely continue to do so in "scary" situations, followed up by the owner doing the same activities and making the same corrections, just hopefully on a slightly less stressed horse!
This horse does give a "warning" but that warning is his general way of going, so not necessarily easily recognisable. He is a very chilled horse generally, so does not give obvious signs of tension, he's just a little bit primed all the time and a small start at something like a squirrel in a hedge can easily be a quick trigger. Tbf, every time he's been "caught" he's done it less enthusiastically the next time and in fact has not spun for some weeks now. (Watch, now I've cursed it . . . )
I did have one that never even showed a tendency until we took him schooling in a place with mirrors! He initially spun EVERY time he turned a corner and faced an image of himself, even down the end of the school, although he was fine with other horses working around him. Proper sit down, turn around, panicky spins. It took the better part of an hour before he could cope and he stayed wary but again, never showed it in any other situation, which is quite unusual.
Triggers can be unexpected sometimes. I would also say your horse undoubtedly has one, but it may be very small and, from a human perspective, not very sensible. In some ways it doesn't matter what sets him off if it's something you meet in places like collecting rings, you have to teach him to deal with it.
It is quite possible that a trainer will be able to fix him for you, by changing his patterns of movement and teaching him to deal with more situations that make him tense. This would probably be money well spent. Make sure they teach YOU how to deal with it, too, though, both so you can keep doing the homework and in case you see glimpses of a relapse.
Don't panic.