How much light does a plant really need?

Say you get a week of cloudy weather , how much will the plant be overall effected? I really wouldn't think a hell of a lot. You get sunburnt on a cloudy day , plants still get UV light on a cloudy day. Maybe it won't grow as fast which I get but if you arnt racing the clock then the issue should be small shouldn't it?

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

shit when ever im out visiting my plants its going to turn into a cloudy day for them haha ..
atleast while im out there who knows how long it will linger around

guess it all depends on what kinda clouds your talking about