A way I use timeboxing is when I have a task or project that I wish to complete, but I don’t really know where to begin, or it seems like it’s going to be a long time before I can finish a meaningful chunk. Or maybe it’s something I find really tedious and would have a tendency to procrastinate on. Then I use timeboxing to simply commit to working on the task for a given period of time to make a dent in it.
I normally use a period in the range of 30-120 minutes. I release any concern about reaching a particular milestone within that time — I simply commit to putting in the time, regardless of how far I get.