What's wrong about billing hourly? Sometimes a client ask me to check something that's not working as expected, it could take less than 10mins, I won't do it for free nor bill a week, I'll bill an hour. I'm happy and he is. If it takes longer I go with half-day, full-day increment.
I do document parsing/data grooming so it's a lot of tweaking/fix as the client do the Q&A on the data.
Actually I overbook myself and offload some work to reliable part-time employees (the client is happy to known that not only me but other people are working on the project).
It may be counterintuitive, but sometimes it's better to bill nothing at all for something like this than to get involved in the minutiae of billing in small increments for specific tasks. This enables you to remain psychologically anchored with that client as a daily or weekly high-value consultant, and doesn't undermine your ability to maintain an optimal billing rate and substantial minimum increment.
I think that's a good scenario for a retainer. You have a long term maintenance relationship set up that typically doesn't require chunks of work at a time.
The overheads to doing a 10 minute fix are massive: they email/call you to make a request, you change work contexts, fix the issue, test and release it, notify them that you're done, keep track of the time you spent working, send an invoice at the end of the month, keep an eye out for payment, thank them for paying etc.
Rounding up to the hour mitigates this, but unless you're doing several maintenance requests per client per week, or are charging very high ($250+) hourly rates, your business is probably losing money by keeping this client on the books.
I think it's better to negotiate a monthly retainer that ensures making tiny updates is worth your while, and then just have a set-and-forget invoice that gets sent automatically every month for that amount. Even better if you can get paid by direct debit.
I do document parsing/data grooming so it's a lot of tweaking/fix as the client do the Q&A on the data.
Actually I overbook myself and offload some work to reliable part-time employees (the client is happy to known that not only me but other people are working on the project).