1: Keep it as long as you want but make sure ph/sg
/temp is still the same as the main and if you don't have ph running the whole time at least mix it up for a couple of hours before you use it to oxygenate it again.
2: I top off daily to keep the sg
from changing too drastically. If you haven't been able to top off daily (gone for the weekend for example) then top off half the usual and the other half a couple of hours later. There is no set formula since tanks evaporate at different levels due to the environment, lights, temperature, ect....
3: Higher for reef tanks because it's closer to natural ocean salinity levels. With a higher sg
/ph changes can be more dangerous and need to be adjusted/monitored more closely.
Reason for keeping it lower is less salt use, less stress on new fish/mobile inverts to acclimate, and leaves a little more room for error i.e. heat fluctuations, sg
rising/falling too quickly from evaporation, ph fluctuations. ect...