Hi pullinup,
The maximum restriction on ready-go tournaments is 12 hours (from the start of the 50th entry), however, if all 50 players complete the round before the 12 hours is up then the tournament will automatically close. I've heard of a few instances where some players have been able to do 3 or 4 St. Andrews ready-gos in one day's time.
The time was recently reduced from 24 hours to 12 hours, but we're hesitant to go much shorter than that. With a shorter timespan that means the top players on this site will be able to get into more tournaments which means tougher competition for everyone. With the 12 hour rule the best players are spread out more evenly giving more people the opportunity to cash in.