Whenever you are in a base with at least one power generator and at least one operational oxygen generator, your suit seems to fill at a constant rate.
The refill rate doesn't seem to care if your base is at 50 oxygen and dropping rapidly as you suck it in, or if your base is at 0 oxygen and the generator is severely overloaded. Notably, once your suit does fill up, the base's oxygen rises very slowly; quite inconsistent with how fast it was dropping to fill your suit and how fast the suit filled when the base was dry.
(Roughly, it looks like you need around 200 stored oxygen to refill completely from an unpowered base, but only about 25 generated oxygen if you start from an empty base you've just cleaned the panels at)
Suit power is the same way but more obvious; a fixed recharge rate regardless of whether your base has severe blackouts at 5 gen / 35 use, a small surplus, or large and charged battery supplies.