cab ride time to Laguardia (rush hour)?

I would allow myself AT LEAST two hours during a weekend rush hour. First! You have to find a cab and pray that the streets leading to the Midtown Tunnel (or FDR North to the Triborough Bridge) aren't log-jammed. Then! You must really pray that the tunnel/bridge and LIE/GCP aren't packed like sardines during rush hour as well.

You might also consider taking the N train to Broadway in Astoria and picking up a cab there - there are lots of cabs in that part of Astoria. You miss Manhattan traffic that way and save a bundle. You could also take the 7 train to Roosevelt Ave and bus/cab/car service from there. I've known a car service to take three hours to get from Grand Central to JFK in rush hour because the driver avoided the expressways - that was an excruciating journey.