Daily, either in the moment, or a little after.
I used to keep a chart on my work computer as a word document, basically adding in what I ate for breakfast, lunch and any snack. The next morning, I would add in the previous night's dinner. Of course, that was just tracking meals, and I would add what diabetes med I took with the meal, and I made a note of glucose readings also.
Even if you don't have access to the points information right away, writing down the food keeps you on top of it, and you can add the points in later, for instance, print it and take it home for the night.
Currently, I'm tracking online, and I do it a few times a day when I can. It isn't hard to remember at 3 o'clock what I ate at noon...
I don't have an app phone; if I did, I would definitely use that.
It isn't so much the tracking/writing, its calculating the points...so you know what you have left to use.
Make it a priority. You have pen/paper at least.
I don't know what the onliners get, but if you go to meetings you get a blue points tracker book so at least some of the information is right there.