I leave my plants in as long as I can, but I have to pick all the toms before a frost (in your state, you probably don't have that problem

), or else they won't ripen, but will just rot. You say they are producing well, but they aren't changing color. If they have at least a little color to them, you can pick them and let them ripen inside. If it's a choice between having a plant producing a lot of green tomatoes, or no plant at all...I'd stick with the plant.