Depends, but mostly 40 mins unless it was with a really good person/ a full training day,I would travel up to an hour and a half for that kind of thing.
I did travel about 100 miles but roadworks on motorways means it now takes so long it's not really doable. Horse is knackered and I've had enough by the time we get there!
I travel about 50 minutes twice a week! I would probably be prepared to travel 90 minutes for one I thought would be really useful, but only occasionally!