How long can you keep a opened bottle of wine?

[curious]

I'm no expert, but as hubby and I don't drink wine fast, we have leftover wine.
We have found that it varies from whites to reds. One thing that we do is put the cork back in (naturally) and then wrap it in foil, that seems to help it last longer. We also incline the bottle, not totally flat, just enough so that the cork can get some wine on it.
Hope that helps, others here are much more knowledgeable than I am.
Janette