Allot yourself a certain time of the day to play them. Basically, schedule your play time and only play during that play time. That's what I do. I play at a certain time of the evening, for 2 hours maximum. Otherwise I don't really touch my games. If I'm doing something else at that time that I feel takes priority (such as working on something or doing something else with friends or family) then I'm probably just not going to play that night. That's basically how my friends and I helped get one of our group out of a gaming addiction: hanging around with his brother at their place doing other stuff. Over the course of a few months he slowly would come out and make his way closer to us or hang around in the doorway of his room watching us for increasingly longer periods of time, until he eventually would actually log off the game and join in our activities. We just consistently got together at the same time every week. Nothing more to it.

If there is nothing I need to do that day, I don't see a problem with playing for a long time

Be careful with games without stopping points. Also be aware of the games that engross you in a way that you just lose track of time, play them only when you can get lost like that.

Find a hobby to do on the side that you can use, maybe something with progression so you have something to focus on and track progress to keep you wanting to do that and not just stick to playing games. Something like drawing, learning an instrument, maybe even exercise, whether it be standard gym stuff or something else like skating or bouldering. If you still want to stick around a computer for whatever reason, maybe learn to code? Having a creative outlet will help loads to keep you from games if you have troubles endlessly playing them because you start getting the creative itches that makes you want to get something done related to that hobby, instead of just procrastinating.