Ot depends on the client, e.g., if they generate relatively small tasks I'll go in 15 min increments.
I rarely have things that take that short of a burst, but I'd be uncomfortable billing someone for an hour of work if I only worked 15 minutes. That seems dishonest to me.
That said, I bill for the time it takes me to get set up to do that work, e.g., if I have to spin up a specific environment, or gather reference materials, etc. that is included in billable time, because it's time I'd normally be doing something else.