hi ya,
I damaged my voice about 6 years ago and couldn't talk for 6 months! So the answer to your question, of when will you know if its safe to singing again is, you need to get in touch with your singing instrument, you will know when you are ready as it will feel fine, a little dull throbbing no probs, any sharp feeling then back off! (You will know what I mean if you feel it!) just take it easy, try vocalizing on NG as in sung, this is a nasal constant, try sliding it gently down your range to start with then up and down, gently going a little higher every time, until you reach the end of your range, another good one is to sing in falsetto to start with as this is great for healing the vocal folds as it encourages them to work on the thin edges, make sure it's not to breathy. When you do this correctly for a couple of minute your throat should feel great and when you speak it should sound nice and clear without any croakiness.

    hope that this helps?

            cheers Dylan   www.vocaltechnique.co.uk