Each training center has a limit of how high they can take your characters. Without looking it up, I can tell you that there are three In Might and Magic VI where I generally don't reach the limit before winning: Blackshire, Castle Ironfist, and Paradise Valley. One is limited to 100, one is limited to 200, and the other is unlimited. (Again, without looking it up, I think it goes Blackshire 100, paradise Valley 200, and Castle Ironfist unlimited, although the last two may be reversed). The only real level cap in the game is your own patience.
With the other two games my memory's a little fuzzier, but it usually depends on the level of the area you are in. In Might and Magic VII, I think your really high level training will be in the Pit and Celeste. In Might and Magic VIII, I remember the Isle of Regna seemed to have an unlimited training center. Again, no real level cap.
Sorry, there used to be a place I could supplement my memory by looking it up, but it seems the information has since been taken down. But with leveling you should have no problems. Once you're past the point where you have more experience than money to take advantage of it, you'll find it very easy to get to these training centers and pay the amount of money they charge.