Depends on a lot of things. Is the new engine complete. A lot of spare engines need fitting out with IDG, Hyd pumps etc.
But my record was in the Gulf Air hangar at BAH. I came into work at 1400 and everything was ready. The new RB211-524 was complete and in the hangar, The engine change team was waiting, and the TriStar was just being towed in. I had a good team of mechanics, all local Bahrainis, and they were experienced. They changed an engine every week. It was a wing engine and we used a bootstrap (we didn't have a 10 ton hangar crane). The aircraft was out on the run bay at 1830. We then had to do all the runs as the engine had not been test bed run. (In those days GF changed modules in BAH but had no test bed.) So it was tests 1-17 incl., and we had it up on the ramp at 2015 ready for a 2130 departure.
To change a RB211 you need about four technicians and a total of about 12 people. The other 8 assemble the bootstrap equipement, raise the stand, lower the engine, raise the new engine and then lower the stand and dismantle the bootstrap gear. A RB211 weighs about seven tons, and we pulled it up with chains, so a lot of energy is expended. A Centre engine took about 12 hours because everything had to be lifted up into posn.
Away from base it will take a lot longer.