+1 on what Daniel said, though IMO it's kind of a waste to wet hop for bittering; the big advantages are in the volatile flavor/aroma oils. On the other hand, if you grew them and have plenty, there's no reason not to go crazy (though if you're not measuring the AA% on your homegrown hops, that's another reason to go with commercial hops for bittering). I have heard some brewers say that overdoing it with the wet hops can get really grassy tasting, so that's something else to consider.
I've always used 4-5 times the amount as dry, which is in the same realm as Daniel's figure. I like doing a wide range of additions from 20 to 0 min (say, some at 20, 15, 10, 5, and flameout) to cover all the bases on the volatiles.
Note that some of the moisture in them is going to come out into the kettle; if you're particular about hitting gravities exactly, aim a point or two higher than usual the first time you wet hop, keep records, and adjust from there.