Yeah, don't get your hopes up. Do you really think the Port Washington service will increase to every 15 minutes...especially with the financial problems the MTA is facing? Does any other LIRR route operate that frequently?
Besides depending on that highly unlikely occurrence, your half hour estimate bw LGA and Penn Station would depend on having NO wait for the AirTrain or for the connecting train into Manhattan. If you have no wait for the Q70 bus and no wait for the E train at 74th St., you'll be at Penn Station in half an hour.
Believe me, this AirTrain is not going to save any time -- except, maybe, if you compare a best-case scenario like above to a worst-case (traffic, delays) on the bus/subway combo. AND it's going to be more expensive for passengers.
AirTrains are great for hard-to-reach airports, like most others in U.S. (and NYC area). LGA is not hard to reach...geez, you can walk to it from like 5 different neighborhoods in Queens!