for this specific task, i sell batched hours, usually in 8 hour blocks (being a full working day). i always say upfront how long i think tasks will take, and re-invoice when there is 2 hours or so left, depending how quickly the client burns through the time.

this is the only one that has worked for me and kept my clients happy too. i used to do monthly billing, but clients often felt like they were paying more than they should, and the reality of it is that with the batch of hours, i end up invoicing more frequently and getting paid more, for the same workload.