At this point in Auggie's career we do private lessons maybe once a week when the weather is nice, as in not too cold and not too hot; otherwise we just practice on our own at home. It's nice to get him out on ALL the equipment as well as have somebody else watch us work so that's why we still do private lessons.
The winter is typically our off season from trialing and practicing so we do smaller drills and stuff that we can do in the house. The weather this winter has been unseasonably warm and very little snow but the ground is still CRAZY muddy; last year he slipped in the mud, face planted, and wrenched his neck really good, so I still haven't done much outside with all the mud just for safety reasons. I don't need to break my dog for a game. So even this winter we've been mostly inside doing smaller jump work.
I'm off to a trial this weekend, but we haven't trialed since the week before Thanksgiving in November. Then we have a weekend off, then we're off for four days at Louisville. I think I have two in April, and maybe two in May also? Then back to about once a month during the hot months, then it usually picks up a bit in September and October, then finish with the one trial in November (though last year I skipped that one because it fell on the same weekend as Thanksgiving!) Then off for winter.
Of course this all depends on money too. If I can't afford a trial (plus the gas and hotel that comes with it) we skip it. I usually only do Saturday and Sunday because I don't have a lot of vacation days from work, but this year I got moved up to the next tier so I get an extra week, so I will possibly be doing more Fri-Sun trials. I also typically don't travel for rally or obedience. I hit the local ones if I can (for their fall obedience trial I was in Indy doing agility) and if I'm going somewhere they also offer ob or rally, I'll enter there too. I'm not doing it in Louisville just because I decided it's already such a crazy weekend and I didn't want to make it any more stressful for me or the dogs... plus I haven't even sent in Payton's papers yet so he has no registration number for me to use. =P
When Auggie was just getting started we did weekly classes, one night a week. At that time I worked days and went to school at night so I had to try and juggle my college classes and Auggie's agility class around. We didn't trial nearly as much then... we started just doing the three local trials, then the next year I think I did maybe six, then it kind of exploded from there.
Payton has only done one round of puppy classes and will eventually do more, so he'll end up in weekly classes at some point for probably two or three years. I am lucky that now I teach where I train, and his puppy class was free... and I probably won't pay full price for his agility lessons either if I pay at all... so that will definitely make some of that easier. Once I get Pepper back I will put her in a class too, I think she's absolutely ready for a class. I've already done a couple of private lessons with her at the big field but I think she would benefit from being in a classroom for a while with other dogs and people around. So same story with her. If I'm lucky I might be able to have them both in a class on the same night... Pay at 6, Pepper at 7, something like that...
From there it's just a matter of affordability rather than time, like Laur said. Trials aren't cheap and they keep going up, and there's hotel costs and gas to factor in, and once I'm trialing multiple dogs, well... it's likely my frequency will drop once I'm entering two (or three) dogs instead of just the one.
ETA: I will say that I'm basically training my dogs all the time. A lot of the stuff we do just in our day to day translates into the ring. I also do a few 10-15 minute sessions during the day; usually one when I'm home for lunch, a bit right before dinner (or they work for their dinner) and then late at night before I start getting ready for bed. Like my last "official training sessions" are at 9pm. I am casual about it, I don't clear out hours of time for it. Just little quick and often intense sessions.