Help... personal finance issue...?

I am 100% positive that I will miss at least 3 payments in a row on a $28,000 line of credit w/ a minimum payment of $500. It's a unsecured line of credit. I have contacted the creditors etc. and of course they have no way they can help me etc. My credit is a 660 now. If I am paying everything else on time, how bad will this hurt my credit? Until now I have never made a late payment on anything. Question number 2 is how long before the creditor can send me to collections or file judgement?