Don't think he's retiring yet but I think he just needs to release one final album after this one.
He's 41 now, will probably be 43 or 44 when/if he puts out another album. If he can release one more album that's as quality as MMLP2, his legacy should just strengthen that much more. I expect to see him try to get into more acting and lots of producing after he's officially done rapping.
Music is in Marshall's blood too much to not be spending lots of time in the studio even after he's done rapping.