How often is enough?
Just like with regular sports, ITG requires practice and training. It's an activity that benefits from progressive overload and it is very easily to quantitatively measure your ability:
BPMs
Volume of streams (in measures), or in time played
Timing accuracy
Numbers and statistics are a great way of objectively measuring progress, so just like training in a gym with barbells and measuring gains in terms of poundage, ITG is much the same way.
Players get better by playing regularly, but how often is enough?
Some players are able to play every day for many weeks on end, and others play every second day. Some are gifted and only need to play once a week to make small gains but they can maintain their level of play by playing very infrequently. Some are hard gainers and have to play intensely for a long period of time to start realizing any gains, whilst others improve on a session by session basis. This is all likely due to the way the body is trained, but also has alot to do with genetics. ITG favors Type IIA muscle fibers generally (songs typically last between 1:30 and 5 minutes) and have moderate to high activation speeds (up to 16 activations per second).
Some players excel at slower stamina but struggle with short bursts of speed, whilst the vice versa does exist (although it is much rarer).
There's such a thing as too much
Overtraining is definitely a thing in dance games. I attribute most of it to a lack of proper knowledge and understanding on the part of the player. Remember, dance games is not a professional sport, so there are no sports physicians or experts who are modelling training regimens for athletes to follow.
Players simply play whenever they feel like it, and rest whenever they need to. However, some of the more aggressive players try to play more than they can comfortably handle, and end up pushing themselves a little too hard, one too many times.
It doesn't help that players also consume the wrong foods and drink alot of the wrong drinks, so their bodies aren't receiving the fuel it needs to properly recover itself after a hard session. What ends up happening is that the player will return the next day without proper rest or recovery and attempt to play a similarly difficult set, then underperform and become frustrated with themselves. This scenario is all too common, and I fall for it from time to time in my zeal to improve, but it is not unheard of that back to back sessions on consecutive days can sometimes yield some freakishly good results, and many of my personal bests have been attained under similar conditions.