What if I don’t use all of it — does it roll over?

Unused allowance rolls over up to one month's worth, so allowance plus rollover never exceeds about two months at once.

Yes, up to one month’s worth. Anything beyond that doesn’t keep stacking, so your allowance plus rollover never exceeds about two months at once — a deliberate cap so a quiet period can’t build into an unexpectedly large bank.