Honestly I don't see RDJ hanging up the helmet for a while. Sure they may go 'yeah we're done' but there's a dangling 'For now...'
Personally I wouldn't mind even if there weren't any more "Iron Man" movies, if Iron Man and Pepper show up in other Marvel movies. Sure there are plenty of Iron Man centric stories to tell but honestly I think at most there could be another trilogy before people people go "Yawn... Iron Man beating up another armored guy... again."
I think it's a better idea after IM3 to focus on building up the rest of the MCU. Like with Avengers, you got a little bit of everyone - save for Rhodey but - I think RDJ can fit in a few shorter things than more massive movies.
Of course, it depends on how big the truck of money they roll up to his house is. Granted he isn't getting any younger and eventually they'll have to go a bit younger - but RDJ is sorta Tony Stark at his core. Rob Lowe... maaaaaybe but I think people would go "Okay where's the REAL Tony Stark?"
Only thing I can think of that would let RDJ play Stark for a longer period of time, regardless how much he ages, just not have him get out of his armor. They can put in some Just for Men in his hair and mustache, make up and CGI out any wrinkles and just use the HUD for most of his facial scenes.
Hell for most of the Civil War and Secret Invasion arcs he hardly EVER got out of his armor.