crankshafts are probably the most durable part of the engine....
The parts you will most likely need to replace are:
-Piston & Liner
-Connecting rod (integrated bushings)
-Front & Rear bearings
You will know when the piston and liner need replacing, the compression will drop, the engine won't have the "edge" it once had, and idling may be unreliable.
I always replace the conrod at the same time as the piston and Liner, as the bushings do wear and get sloppy.
The crankshaft and wrist pin/gudgeon pin are made from extremely hard steel and should last almost indefinately if properly maintained.
The bearings can last a few rebuilds, if they are kept clean and no dirt has been able to enter...
You can't really put a label on an engine stating the life, it will vary with fuel used, application, maintenence, etc.