How much light does a plant really need?

Think of growing outdoors in a perma cludy climate as opposed to a sunny one as natures equivalent of growing with cfls instead of hps.Yes it will grow but it will be that much worse than if it was grown where there is a lot of sun.A few cloudy days here and there won't matter but if you live somewhere where theres no sun your not going to have the best plants .. you could still end up yeilding better than if you grew inside ..you wont get 3 lb plants but half a pound to 1 pound i think could still be achieved in a cloudy place.

edit oh you say a cloudy week ..not an issue .

weed can grow quite well with minimal light, but wont produce as much