How long does it take to visit all (or the most) of British Museum? I arrive there on Sat. and leave on Mon.?

Answer 1

Impossible to answer.
Do you run through the exhibitions, or do you study each item.
If you run you might be able to see all exhibitions in a day but will not remember much.
If you study all items you will never be done.

So allow yourself a set time and enjoy that to the fullest. The museum is there to come back to.