They are most likely going to use the same system present in combos to introduce regimens this time, probably tie it to a status effect as well which 'tracks' which effects the monster suffers in a given amount of time.

From my perspective on how they introduced combos with a status timer, it seems to be an actual programming shortcoming rather than a planned feature. However, given this is likely a status on the monster, not on you, you can probably notice when you've ran out of time to get the Regimen going, but not how much time you have left.

Also please, please, PLEASE don't make it a completely arcane and unexplained system. It's all nice and cool to have some combos to discover by yourself and all but, you know, at least some introductory and basics, and some advanced 'iconic' ones, needs to be obviously executable, or at least there needs to be INSTRUCTIONS on how it works. Otherwise people will nevr really get the hang of it at all.