Digestion time?

My husband and I are trying to housebreak our 4 1/2 month old female JRT. According to most of the puppy books I've been reading, as well as practical advice from other dog owners, within 30 minutes of eating a meal, my dog should poop. So after 30 minutes (and sometimes even after an hour), I take my dog out so she can do her business. She will do #1, however will not necessarily do #2 within that time period. We're outside for about 20 minutes, and she does not always go. Sometimes she does have to go really bad(the signs are very obvious looking at her behind), however, she will also hold it. The questions are:

1. How long does it really take a JRT to digest his food and have to poop afterwards? I get frustrated taking the dog out for 20+ minutes and nothing happens.
2. Is this a behavioral problem or is this just the way my JRT is and she likes to hold it?
3. Is this normal?
4. How long is it safe for the puppy to hold it?