[aft_lizard]

My bad, I was off on the time it actually takes as little as 30 minutes to have problems.
http://digestive.niddk.nih.gov/ddiseases/pubs/lactoseintolerance/

[raybbr]

People who do not have enough lactase to digest the amount of lactose they consume may feel very uncomfortable when they digest milk products. Common symptoms, which range from mild to severe, include nausea, cramps, bloating, gas, and diarrhea. Symptoms begin about 30 minutes to 2 hours after eating or drinking foods containing lactose. The severity of symptoms depends on many factors, including the amount of lactose a person can tolerate and a person’s age, ethnicity, and digestion rate.

"Symptons begin..." That doesn't mean diarrhea in 30 minute. Still you would have to clear any food already present. In an extreme case? Maybe. I know from personal experience that it takes about four hours for the diarrhea to kick in. :(