Depending on how much hard driving or wheeling you do, I would recommend a change once every 15,000. If you do a lot of deep water crossings, I would service them after each trip or at least give a peak inside to make sure the fluid isn't milky.
As far as capacity goes, you need to fill till the fluid level is sitting at the base of the axle tubes. You can make a dip stick out of a bent zip tie to check where you're at. The PR60 will have a port on top at the pinion and it should be filled first and then finished up at the diff cover. The PR44 won't have this port.