leaving game
The system already handles if a player leaves the game completely. If they disconnect, they have a certain amount of time to reconnect or the game is lost by abandonment. The amount of time is dependent on the overall time control.
What the site doesn't have is a per move time counter. So, if a player doesn't disconnect and just lets time run, there isn't anything that can or will be done. People are allowed to use any time allocated to them and there is no real technical way to determine if a person is having a long think or is just making the opponent wait.
However, in the latter case the system tracks people that do it a lot. They get flagged and if a certain percentage of their games end like that, they get restricted so that they have to wait to start new games.
https://support.chess.com/customer/en/portal/articles/1444922-fair-play-policy