I will either do exactly like what you suggested, "process 20 bookmarks"

or

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.