I've done some data logging in my bathroom in the morning when I get up to shower. I've seen a 15% increase in RH in about 3 minutes. You could just look for a sudden change compared a running average. Not sure how often you should sample, but I think I was sampling about every 10 seconds. I also used an RTC in my project -- DS1307. My temp/humidity sensor is a Sensirion SHT-11.