They don't top it up. You get one extra day on the 1st and one extra day on the 15th to replenish those you use. I think in the past, they may have actually added time when you renew or purchase a premium membership but that hasn't been done for some time as far as I know.
https://support.chess.com/customer/portal/articles/1444787-how-does-vacation-work-how-much-time-do-i-get-
It says you have two months of vacation. When will chess.com top that up again?