There are apps that block opening apps longer than some amount of time. I used that to reduce my Reddit usage to a healthy level before I eventually bailed on it. Or you can use the simpler strategy of setting an alarm for yourself (e.g. if you want to play an hour, set an hour timer when you play).
If you find you're consistently going over your time limit for a given thing, stop using it and replace it with something that you have a healthier relationship with. At a certain level, it's an addiction, so cut out what you're addicted to and replace it with something else that's interesting. For example, if you like shooters but Splatoon is sucking too much of your time, maybe play Metro, Superhot, or Wolfenstein, each of those is SP only and has a clear ending, so you're unlikely to get addicted to it.
I switched largely to SP games, and now I'm much happier and have a healthier relationship to games. The same is true for other things I used to spend way too much time on.