I agree with the others. Start out slow & gradually increase the length of time.

With Kaige I started out doing about 5, 10ish minute training sessions a day. As he aged we increased the length of time, but I found out with him that as we started doing the 30 minute training sessions he didn't need or want 5 sessions a day anymore. We now do 15-30 minute training sessions about 2-4 times a day. (4 times a day if each session is 15 minutes long, 2 or so if we have longer sessions).
You'll be able to tell by how much they are learning & how quickly they get bored with it.

But of course we also follow the NILIF rules (or at least try to most of the time) so we actually have a thousand, one minute sessions a day. lol