It really depends in the wine. An old cab and some good Pinos can take a day just to open up in an open full bottle. Most of today's wines aren't really built for aging or breathing though and begin to develop odd tastes after a full day exposed to the air. The only way to really know is to take a sip and see how it tastes. You can increase the amount of time the wine is palatable by keeping it in a cool spot and transporting it to an airtight container (be sure the wine completely fills the container to reduce the amount of air in the bottle)
There are many different factors that take place when deciding how long an open bottle of red wine is good for. For example it depends on the year of the wine, the type of wine that it is, etc. Most people usually play it safe and only keep it for 3 days after opened.