Usually they are hibernating by now. it might be too cold to let it go now (it is already here in NY state, and I imagine it is chillier up there.)
You could choose to keep the frog for its entire life. It might not be a good idea to release it when it is fully grown next Spring, as it will not have had experience with hibernation in its first year, and might be really confused. These frogs do hibernate as larva/adults, and some even go underground ass tadpoles for the first year of hibernation.
Green frogs are great as pets. I have one now and he has been mine for over two years now. Simple 20 gallon (long) tank setup, half land, a few inches of water.
Maybe you could let it go on a sunny day? It would have to be at least 45-50 degrees out though, and that might not happen. Let's see if anyone else on the forum has any advice on this.
-----
*~Ginevive~*