20A x 20 hours = 400 Ah would drain battery from 100% to 0%, whatever voltages the battery makes at those SoC at that current.
See technical manual for curves.
Some people say don't draw below 50%, but I've planned around 70% DoD (30% SoC) before disconnecting loads, and down to 20% SoC keeping inverter alive (because PV is AC coupled when the sun comes up).
I have one bank that size 100 Ah @ 48V, another 400 Ah @ 48V.
The bigger one can only get my home through the night if a number of unnecessary loads are shut off. (Several older refrigerators are probably the main drain.)
Here's the manual for mine. If you can't find the detail for your brand/model then this could be reasonable to use.