Obviously, it depends on the type of load being loaded/unloaded.
But let's assume that it is a full load of general cargo with no offsize, and a reasonably proficient Handling Agent.
Utilising both ends of the aircraft I have seen it done many times in one hour. But, using the side door only, your are looking at 90 minutes or more.
That is a reasonable rule of thumb, but there is no standard as too many variables come into play.