"Another alternative is to treat the 25m as a minimum time in Pomodoro: if you are tired or have reached a natural break point for your task then stop. If you're in the flow continue."

This. The pomodoro technique has helped me greatly, but the time limit is really just about motivation to get started (in my case). It helps by allowing me to tell myself: "You can do this... you can do anything for 35 minutes (the time limit I prefer)."

However, it is very often the case that once the timer expires, I just turn off the alarm and remain focused on my work. Sometimes my session lasts up to an hour or more. And of course, at other times, I finish the task I wanted to work on before the timer expires because the level of focus/flow I was able to achieve enabled me to finish the task more quickly than I thought.

My story is just one, of course, but I highly recommend at least trying the pomodoro method or some variation of it, just to see if you have similar success.