There are not time limits, just whatever is required by your instructor before he/she will let you solo. I wouldn't mess with a pitts, although, it may be fun but the really high performance acro planes tend to be really pricey. I would look for a decathlon or citabria or even a 150 aerobat. I used to teach aerobatics and it is a blast, you'll also want to take it easy at first so that you don't get burned out. Start out with short sessions (like 20 min) and work your way up. It takes a while for your body to get used to the stress of higher than normal g-forces. If you have a good instructor then they'll know about all this. Another way to do things cheaper is to look around and figure out who owns these kind of planes then offer to buy their gas and clean their plane/car in exchange for flying. This is also a good way to get multi time as well.