"are [endorsements] good for as long as I fly or do they expire or have to be renewed often?"
They are good forever - so long as you do the biannual flight review, which every pilot level (Sport, Recreational, Private) has to comply with.
Of course there are other limitations, like having to have three landings within 90 days in the same type of aircraft in order to take passengers, but once an endorsement is satisfied it remains in effect so long as one is current.
"A person who holds a recreational pilot certificate, has logged fewer than 400 flight hours, and has not logged pilot-in-command time in an aircraft within the 180 days preceding the flight shall not act as pilot in command of an aircraft until the pilot receives flight training and a logbook endorsement from an authorized instructor, and the instructor certifies that the person is proficient to act as pilot in command of the aircraft"
So you'll have to fly at least once within every six months or have to have a flight review; I can't find any such rule on Sport Pilots.
The differences between Sport and Recreational really boil down to what kind of plane one wants to fly. The Sport Pilot is limited to aircraft allowed by Weight (less than 1325 pounds), Vh (top speed in level flight not more than 120 kts), and number of seats (two total), while the Recreational Pilot is restricted by horsepower of the plane (not more than 185).
I'm always discouraged when I read about pilots with a Private licence "stepping down" to fly under Sport Pilot rules. Are Commercial Pilots rated for 747's "stepping down" to Private Pilot status when they fly a 172 under VFR conditions by themselves?
Everyone with a Private Pilot license who climbs into an Aeronica Champ under VFR conditions is "stepping down" to Sport Pilot rules. I think they might have a contrary opinion of anyone who would tell them they're not operating as "full" or "real" pilots.
I'm a Sport Pilot not because I'm afraid of the medical (45 years old in good health). I chose that license because it makes the most sense for me - both in the kind of flying I intend on doing and for financial reasons.
In fact, the Private Pilot license holder who allows his medical to expire and flys under Sport Pilot rules is NOT a Sport Pilot license holder - and it serves nobody to confuse the two.