Hi Ahmed, can't help with expected values but my dec 71 plate G26 420i Grand coupe has done 29,732 miles and not had to do the rear pads or a message, it's service due in sept.
 
	it had a service nov 22 with 17k miles and nothing for the pads needed doing then. 
 
	it does sound dodgy that those would need replacement before the front. 
 
	matt