I used a smack pack, Wyeast London ESB, last night for the first time. I thought ahead and broke the liner about 3 hours in advance and it was active. I pitched it around midnight and it was only about 7 hours later that I had to leave my house. I could see some foam forming on the top, but no bubbles yet. I'll see when I get home.
If it takes a little longer to start that's no big deal as long as it completes.
Edit: I'm home now and the beer took off at some point during the day and is bubbling like crazy.