Unlike raids atop a gym, there is no counter telling you when a Team Rocket PokeStop will stay the way it is. They dont last forever, but they will return to normal for the player who defeats the grunt hiding inside.
Its generally understood that a Team Rocket PokeStop lasts around 30 minutes before naturally returning to normal. You can challenge as many grunts as you can find and invasions can happen even outside of raid hours.
