So long as it wasn't hard work, after a few minutes. If hot and sweaty I'd leave it about 10-20 minutes, if they're puffing I'd leave it longer. I'd rather keep walking round if they're puffing, but have had the odd exciting hack where we've come home in a tizz without the opportunity to cool down
I was always taught as soon as the horse is dry (on the basis that resps. should be back to normal by then). So sometimes 5 mins after (by time tack back in box etc.) or up to an hour!
We are only really happy hackers so its not as though I am feeding after an intense long schooling session but I have always fed straight away. I un tack, sponge/brush off as required and then he has his feed. I have been doing so for the last 6 years and he is still here to tell the tale. Im sure other people religiously wait a certain time but I usually only wait the 5-10mins it takes me to get myself sorted!
I've never worried about leaving mine for any amount of time before feeding.
I never put them in the stable blowing anyway and they are fed a mainly fibre diet, even when eventing and working hard, so no issues with eating straight away.
it really depends on what the horse has done, if youve gone for a quiet hack id say it doesnt matter straight away. if you have done light work and again the horse isnt warm/hot id still feed straight away. if the horse has done hard work and is hot and sweaty even if they have walked off abit id wait untill they have fully cooled and no longer as sweaty then id feed.
im not fussy about feeding, as long as the horse is calm and not really hot, sweating, out of breath etc i would just feed after about five minutes or so...depends on the horse and what its done