I have been riding an 8 year old for a few weeks to a month now. She needed tuned back up because she hasn't really been touched since she was 4. I just have been using a basic snaffle on her. I trotted her on the pattern yesterday and it wasn't bad, but how often should I train on the pattern? I don't want to do it too much or not enough. 

On my finished horse I only work the pattern if there is a problem and it needs to be fixed on pattern. Otherwise, I do a lot of long trotting and trail riding to keep her in shape.

You can also "work the pattern" in the pasture, around shrubs, poop piles, tires, etc. 

I will trot the pattern often, but usually only a few times a day, unless I have a problem that I am fixing.  I had a solid, finished horse.  Our routine was that I would long trot him and lope circles to keep him legged up and then finish everyday by trotting him through the pattern once or twice.  I always stepped off of him after we had trotted the pattern.  The barrels were a reward and how we ended the day. This gelding liked the routine and I could keep his feet sharp and moving correctly.  I have a younger horse now.  I will trot or maybe even make a fast loping run.  If she is working correctly, I only work her a few times and then quit and get off of her.  Some horses might not like that routine, but I do it with most horses.

This horse is not finished, we are just starting. 

This is horse isn't finished, we are just starting. I just am not sure how many days in a row I should train the pattern and how often I should skip a day so she isn't overwhelmed with it.

I would ride everyday but spend most of my riding time working on basics. The horse needs to be really broke. That means it flexes at the poll at a walk, trot, and lope. It has a good stop. Can pick up each lead from standing still and work at a slow lope. Horse will move off your leg. When you have that under control, you are ready to start slow work. 

I do my basic foundation work with my 4yr old colt 6 days a week. I try and get her out on the trails1-2 days a week, slow work 1 day (with a snaffle), haul one day with drill/pattern work, and the other two days we are either working drills or the pattern. I try and do a cruise through and push her once a week and then once every two weeks and then rotate that. But it really just depends on what we need. Listen to your horse. They will tell you when they need to slow back down and when they need to be pushed. 

I am not afraid to work the pattern daily as long as it is done slowly and only a few times.  I want my horses to be really patterned and for their feet to go in the same place everytime, but also listening to me.  If my young horse is listening to me, I will trot twice and then put them away.  If I make a half speed run at home, if they work well, then I walk/trot them through and put them away.  I try not to drill them or pick at them.   I almost always do my barrel work at the end of my riding so that if things are going well, I can reward them by putting them away.   Even with my young horses, I make minimal faster runs at home.  I want them confident but not bored.  I am not a professional trainer but have made a few horses with different personalities and this seemed to work with all of them.

I ride more than I train. On green horses.  The barrels are a place in the beginning to rest. Il get them a little out of air wanting to stop. The barrels they set and rest. At each one. Stop set still for a bit. Then go to next one. That way they look forward to it. I'll get off at each one at different times.  Score your horse.  I get them here that have been drilled to death and hate the barrel. If they do good the first time I quit for the day. I may have rode the horse for an hour. Warming them up to  get ready is important.