Personally, I haven't broken past alpha yet (mostly due to various technical or schedule issues), but here's my view on it. Beta is mostly where you start to do detailing and minor changes to gameplay, so try and get your alpha into a comfortable position before trying to work on major detailing. This could be anywhere from a5 to a15 before you think your map is ready to shift into those big boy beta briefs, so try and find a good spot where (in comparison to older builds) people have less feedback to give.
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.
I just move my map, hazyfort from alpha to beta because the number of feedbacks that points out definite flaws of my map has decreased to near 0. By flaws, I mean unfair sightline, one team too powerful in one area and that sort. You can still change small things in beta too so you can go into beta when you are certain that you wont be changing the layout too much.
For me, Alpha is for big experimental changes. When I think the big changes are done and some areas are locked in enough to start some detailing, it goes to beta.
Beta is mostly for refinement and smaller changes while detailing. I'll still make big changes if necessary, though.
RC is when all the detailing and optimisation it basically complete and I think the project is ready to promote and get out there on a bigger scale. Any updates past this point generally are bug fixes and minor tweaks to stuff like health and ammo or timers. Sometimes you still need big changes though. We're redoing a bunch of overgrown.
I personally try to get all major gameplay changes done in the alpha stage and fully detail in the beta stage. I use the beta stage to raise awareness for the map and get in any leftover gameplay ideas. In the RC stage I aim to polish the map in all aspects like performance, lighting, and clipping