I usually get mine done every 6 months but my saddler says some dressage saddles may need every 4 months. I just know when mine isn't sitting properly and call him out don't really go my time but it does tend to be between 4-6 months.
There's no "should" about it really... you don't have to in the first place, it's not a requirement. If you're capabel then look at it yourself every time you ride! However personally, I get mine checked after six months of the initial fit, then every year unles I feel it needs looking at sooner, if my horse has changed shape dramatically or something. I'm not confident enough to look at it myself, and my saddler is just fab and very helpful and if he's coming out to fit someone else's saddle and checks mine and it's ok, he doesn't charge me! It's worth doing every year though for my peace of mind. I suppose really it should be done every six months as the horse changes shape with the seasons. But there's certainly no rule about it!
again, i'd say six months. begining and end of the season.
although i use wintec aith an adjustable gullet, and vary the gullet according to her shape. Also if it is not fitting 100% i find myself on the floor!! (bless, she's good like that!!!)
i generally have someone out who looks at her back twice a year, and checks the fit of the saddle at the same time.
6 months after initial fitting, especially on a new saddle with flocking as it may need re doing,and then yearly..... or if the horse suddenly shows odd behaviour!
For a new saddle or a newly reflocked saddle, it should be checked after one month, otherwise yearly, unless you have a problem or the horse changes dramatically. In youngsters it's best to aim at 2 - 6 monthly as they change so much.