green just means that it still has the natural moisture in it, pretty much the sooner you carve it after you collect it the easier it will be, you do need to think about how the wood will dry out after youve carved it green though. probably the best way to go about it would be to carve it slightly oversize, dry it out gradually in a warm dry place like an airing cupboard, then finish it when its dry.
in general it lasts a reasonable while if youve left it outside, maybe a few weeks. you should be able to feel the difference, green wood will be denser.