So IF that ever happened, it would be the Air National Guard would be the first emergency responder, way before the air force and that would be out of Stewart. So lets say suddenly the portal appeared over NYC, aliens start crashing out. First the alert would have to go through the chain of command to get the guard deployed, lets say minimum of 5 minutes for that to happen. But lets just pretend the COP already got word the order had been given. At that point the first emergency team, who is -suppose- to always be on standby would be rushing to suit up and get to their planes, ground crews would have them already mostly fueled but would have to do a few pre-flight checks while the pilots are finsihing getting read and rushed out to the field. Thats another 5 minutes. Instant take off clearance would be given and 2 jets would be in the air within another 5 minutes (taxi time, final safety check on the current patterns, clearance to go, and relaying of orders and flight plan). Already we've chewed through 10 minutes before a Jet is even in the air. Then the travel time to NYC (90 miles away) even at full burn (these would be f16's launching first so 1500mph) would be another 10 - 15 to get to altitude and speed and get there.
So yeah, 30 minutes? Does not seem crazy. And that's for a first responder, NOT the air force. Look at 9/11 and how long it took to get jets in the air and tracking, even after a scramble call it took 7 minutes to get 2 F-15 fighters in the air, that was with the crew already on alert that something was happening and having pilots sit in ready possition with the planes in full take off mode, awaiting launch orders.
https://en.wikipedia.org/wiki/U.S._military_response_during_the_September_11_attacks