With the dried out rollerballs that I've stuck in a mL or so of water, they soak up about all they're going to soak in about two days, give it 3 to be sure. On a refill without dried up ink "clogging" the works, I'd think it could absorb ink faster than that. On a really dry refill you might add a day or two for the tip to un-crust itself. Or you may be out of luck if it's dried out enough.
The slow part, which takes 4-7 days more with the adding-water procedure is waiting for the dye to diffuse throughout the sponge so that it's writing a dark line again. Not an issue if you're refilling with ink.
I have a lot more rollerballs dry out in storage than I've ever used up by writing with them.
Tombow has a free-ink and feed system more like a V5 or Pilot Varsity. They can be refilled by pulling the point and feed assembly out and then refilling with ink. I find their "fine" points to spill so much ink as to be useless, but the x-fine tombows are a real pleasure and you can check the ink level.
Well, if you have a 0.01g scale, you can check the ink level of any rollerball if you make some notes as to full and empty weight.