I can only tell you that I am constantly at LTN picking up relatives on those Ryanair flights and it usually takes them about 20 to 30 minutes once the plane has landed to exit the terminal and meet me in the pick up point. It has on occasion taken much longer, on the order of an hour. This is all based on no luggage and UK/EU arrival queue.

Were it me, I wouldn't schedule a bus sooner than an hour after my planned arrival just to be on the safe side.