You know how it is - you open a bottle of wine, don't drink it all, and stuff the cork back into it to finish it later.  But how much later can you expect it to still be good?

The simple answer to this question is about 3 days - a little longer if it's a bold, dry red.

Oxygen is the death-knell to wine, turning your deeply-delicious vino into something that tastes more like vinegar - but isn't.  So putting the cork back into the bottle helps - but can only go so far.  The less liquid left in the bottle increases the surface area for the oxygen to come in contact with;  That said, the less full a bottle of wine - the faster it's going to go bad.

This article gives a little more background and information on the subject.