Something to consider is what the party is running away from. Most beasts/monsters aren't likely to give chase, unless they're particularly hungry or have some other motivation to do so. Intelligent monsters might give chase if the party is deep within their lair, but otherwise most will just be happy to have survived the encounter (again, unless they have a motivation that states otherwise).
As for how we handle it mechanically, we play on a grid. The edge of the board is generally considered the end of combat, so once the PCs are off the map, they escape. The enemy might give chase, but that moves away from combat rules. IME, it's not unheard of for a PC to stay behind long enough for the rest to make an escape, but it's not very common.
As for how we handle it mechanically, we play on a grid. The edge of the board is generally considered the end of combat, so once the PCs are off the map, they escape. The enemy might give chase, but that moves away from combat rules. IME, it's not unheard of for a PC to stay behind long enough for the rest to make an escape, but it's not very common.