To be honest, the red light district is frankly gloomy (at least to me) and not particularly pleasant to visit. But if that is what you want to do and by doing I mean walk around (I have no expertise on the time it takes if you have other plans!), it would take less than one hour from the time you leave the station to the time you return there, it is rather small. I'd personally recommend more going towards the Anne Frank house (which is also a lovely little museum) and the canals around that which would take a bit longer but is much prettier too.