Adding in the time for dis-assembly of the engine and re-assembly plus 'cook time' for the mill it usually takes approx 45 min of labor and watching the oven. Just make sure you keep the new ones in the freezer like stated above for at least two hours before you make the swap. Obviously this should most likely go without saying but make sure strip the engine completely, even the exhaust gasket or it will melt. Good luck, post up your results!
PS the easiest way to get the rear bearing out is to tap the mill on a piece of wood. The bearing will come right out. I use a wood dowell to nail out the front bearing.