The primary purpose of warming up is indicated by the term "warm" up. Whether warming up for sports activity or singing - it literally means raising the core temperature of the muscles you plan to use in order to increase the efficiency of the muscles and reduce the likelihood of injury.
As it relates to singing, a warm up routine will also serve to loosen up (like stretching) your vocal folds. Asking tight vocal folds to respond to sudden high intensity singing is a recipe for injury.
Muscles are warmed by increasing the blood flow by increasing the movement of the muscle. As the neurons fire to tell the muscles to contract, heat is produced in the muscle tissue. Once the exercise (such as singing) has ceased, the muscles begin to cool. After 30 minutes of inactivity, a new warm up would be indicated.
Excessive talking after warming up the muscles involved in singing, might delay the time needed for those muscles to completely cool down. So the amount of time elapsed before a new warm up is indicated may vary to some extent depending on how much talking you do between singing sets.
But it is probably a good idea to do some warm up prior to singing again if more than 30 minutes has gone by since you last sang - just to be on the safe side.