Lawyers will be for every 6 minutes worked. Which makes sense, for if they are working at $300 an hour, 6 minutes / 10th an hour - is $30.
Big Four consulting firms usually bill at the 15 minute layer. Their associates, consultants, managers, and directors are trained to the 10th degree to track time, congolmerate work, and are expected to bill at least 40 hours a week. 50 is better.
Freelancer-land? Well, we have what I think is 10 minute intervals. Or 20. I forget at the moment. Anyway, it is highly granular.
How do you handlle a call from a client that takes 2 to 3 minutes? 20 a day is an hour or so. That's time for mortgage, rent, money, investing, living.
I try to bill at 20 minute intervals. If I have a task like a call, then I stop the other work process, pick up with some task I can add into the call, make the progress that need be, immediately enter the time, and call it a win.
So what interval do you bill time in? 20 minutes? Time's a wasting!