personally i would say marinate them for as long as you have - i quite often forget to do stuff like that, and only have 30 mins, sometimes i remember and do it overnight ... again as for how long the answer is just depends! how do you like your lamb done? On the rare side? Well done? burnt to a cinder? lamb is one of those meats you can have rare without worrying too much about poisoning so it just depends - i know this isn't helpful but just until they are a colour you like. The time will depend on your grill, how hot it is, how close the grillpan is etc!