Since beta involves detailing, and detailing takes a while, and significant layout changes may involve significant changes or even deletions of hard work spent detailing, you usually want to enter beta when you're satisfied with your layout and you're done making big changes to it. It may be tough to clearly identify when your layout is good, so you want to test your map plentifully and declare your map ready for beta when you're receiving plenty of positive feedback, and/or you're receiving less and less feedback on the layout during tests. When you've reached a point when you don't know what to change for the next version of your alpha map, you're probably ready to start detailing.