i replaced my front mount and found that it was much easier to shift after the new one went in. its hard for me to say exactly how long it will take though, because i did it in freezing temperatures and took many breaks to warm up.
if it's your front mount that's bad, it's not all that difficult to replace. you need to remove the lower radiator hose and the mount is right behind it. remove the nuts and bolts, jack up the motor a little bit, make the switch, and put everything back together.
if it's the back mount, i've heard that you need to drop the subframe which would take a lot more time and effort.
EDIT: btw, i did that front mount as a 16 year old, so a mechanic shouldn't have any problems.