No real way to know for sure as there are many factors. Overflow setup/depth, depth of return lines, length of drain lines, etc. But in a standard 4x2x2 120g, every inch of water in the display is 5 gallons of water. In my case, I drain down about 3-3.5 gallons when the pump shuts down.
As to your future 55g sump, I'll start by saying it will work just fine. But I will recommend you looking at using a 40g breeder as an alternative. Assuming you're considering a standard 55g, they are very narrow and quite tall if you're using them in your stand. The narrow aspect can also be a limiting factor on skimmer choices.
So, as I said, it will work perfectly fine. I'm only sharing my personal experience with them. Just food for thought and you're welcome to ignore it.