I usually arrive 30 minutes or so before to the first screen show and the rest usually take care of themselves once you are in the theatre. I have done shops where there were 5 screens or so.
The ads appear before the trailers and the showtime listed is actually the start of the trailers. Luckily, the showtimes somehow allowed me to complete the shops. I have never had a movie shop rejected. If the showtimes are tightly closed together, they usually ask you to bring a partner. Unless there was a reimbursable ticket for my partner, I have never had to do it.