This suggestions dovetails well with this: https://feedback.hecticapp.com/time-tracking-feature-requests/p/alert-when-approaching-max-hours-for-a-client

It would also be cool to have a feature where once we go over those hours, it switches to an hourly rate… I know this is how a lot of VAs have their retainers set up. I was able to do this in Harvest just by creating two different “projects”, the retainer work set as unbilable and the OT work set as billable. Once I was over, I just switched to putting my time against the billable project. Still manual, but it worked.

Yes - on my current invoices for retainer clients I always let them know how much of their retainer is remaining.

This would be useful for retainer clients. It doesn't have to be anything fancy, like a graph or anything could be something like:

Time tracked this month: 23 hrs 30 mins

Project hours: 40 hours

Resets in: 15 days