That' s not how it really works...
I mean, you could obviously change your map's prefix to whatever you want, there are no real rules about this subject.
Usually the way people use the prefixes is Alpha for massive gameplay dev, Beta for mostly detail, and minor gameplay changes. And let's be honest, there is almost no way you made it past Alpha in less than a week...
But sure, you can call it whatever you want to. I myself have a beta map that's still under heavy gameplay development... But it's in development for more than half a year.