I will either do exactly like what you suggested, "process 20 bookmarks"
I will schedule in time. So I will schedule in 30 minutes to "process bookmarks"
So what ever I get done, it might be 10 bookmarks but then it might be 30 is not as important as just spending 30 minutes focused on the task.
if I choose to do the time slot approach what i will also do is mark out how many bookmarks was I able to process in that 30 minute block.
so if I were able to process 20 in 30 minutes, and I have 100 bookmarks total.
Then I know that for the next 80 that are left it will take me 2 hours total approximately.
and I will schedule this in 30 minute chunks where I figure I will be able to do 20 bookmarks in each chunk.
This process works for me.