Here's a third vote for 'daily'.

The archive system I came up with requires management approval to move old stuff off so it only actually changes every few months. So really, just backing up after every archive operation would be enough.

But our backups run overnight, there's no load or user interaction to speak of then, and so slowdowns and such don't matter. So, I do do everything all in one shot, every night.

If it ever gets to be a problem I'll reduce frequency but so far nothing bad has come of it.