^ Exactly! There's no real time frame on when you should start selling your beats, it's all up to when you believe you're ready to sell, and if you believe the produce/service (your production) is up to scratch.
Id say first contact some artist/rappers/singers, whoever your focusing your music towards, sending them some beats and asking for opinions or critiques. If you feel that your producing skill level isn't up to par yet, then it'd be better to focus your efforts on IMPROVEMENT rather than on quick money. Treat your music like a craft, and you will get good enough to sell your music for money.
Also I agree with what Cyko said about not worrying about other producers opinions. Of course it's always good to get critiques, but at the end of the day you're not making music for producers, you're making it for artists, and listeners. Producers tend to have an over critical ear when listening to music, and some fall into the habit of over-dissecting music, rather than just letting themselves feel it out.
If your music is good enough to bring out a feeling from your client, then you're ready to sell!
-Stuart B