The Jersey shore, depending on where you go, is pretty far away from EWR.
In any case, it basically depends on traffic. Because it's a weekday, rush hour should always be taken into consideration. However from 12-3 I would assume most people would still be in work, perhaps out to lunch, but not so much to cause a major disruption. Now, I just did a quick search on MapQuest, and they have a route that they say will get you there in 51 minutes (this does not include variables like traffic and accidents along the way, obviously). Personally, I wouldn't risk it, however I must ask, which airline are you transferring to at EWR? If it is through CO, be very aware that as EWR is CO's hub, terminal C tends to get a bit, well...messy. Basically, if you haven't been there before, flights are bundled into lines, so you could be forced to wait with people going on up to 5 different flights. As Cherimoya suggested, you might want to simply consider CO from LGW. There's probably nothing worse than getting stuck in mid-connection.
One side question: How do you intend to get from JFK to EWR
Also, here is the Mapquest link incase you're interested:
Link