If you're driving, and there's no traffic, figure on 90 minutes (minimum).
Here's the way I'd go... take
I-95 south towards New York. Just after you cross into NY, follow signs for the Tappan Zee Bridge
I-287 west.
Take 287 for about 3 miles and follow signs for the Hutchinson River Parkway south. Following this route takes you off 95 and all its pitfalls.
Now take the Hutch southbound towards the Whitestone Bridge.. This highway is cars only. I-95 can go 'down' 24/7. Believe me, I know. If there's a problem with 95, you'll save loads of time taking the Hutch.
In about 10 minutes, bear right at the fork in the road. Do not follow signs for the the George Washington bridge.
Bear right at the fork.
The Hutch dead ends at the Bronx-Whitestone Bridge. Pay toll.
After you cross the bridge, bear towards the right. You should see signs for LGA. The airport is 10 minutes away after you cross the bridge.
dh