Use a timer whenever you are trying to do “meaningful and focused work”. Set it to 20-25 minutes, whatever duration is comfortable for you.
Make sure to put your phone on airplane mode when you are working. You don’t want to get interrupted.
Use a kitchen timer, web-timers like google timer etc to keep track of the time. Especially if using your the clock on your phone is distracting. It’s important that the timer itself is not a distraction, but a focus-enhancer.