Granular pre-emergent doesn’t need to be watered in, although most products work best with at least a quarter-inch of rainfall or water within 2–3 days. Watch the weather after applying pre-emergent and break out your sprinkler if you don’t get at least a quarter-inch after three days.
Liquid pre-emergent is a different story and needs to be watered in to reach the soil. Once again, a quarter-inch of water is all it takes, so you could get lucky if there’s rain in the forecast. Otherwise, running your sprinkler for 15–20 minutes for 2–3 days after applying liquid pre-emergent should do the trick.