How long to rebuild a credit score of 550?

5 monthes ago I was refused a car loan on the premise that my credit rating was 550. I have 1 credit card that has a balance of 8000 (maxed), and i have been making the minimum payment for the last 5 monthes. I presume the low credit rating is a result of 4 or 5 missed payments earlier this year on the card. After being refused a car loan at a major bank, I did manage to find a borrower that would give me a car loan at a staggering 29% interest rate. I have been making the car payments for 5 monthes. Therefor my bills include my car payment, and credit card payment. All of these have been on time for approx 5 monthes, and i'm wondering how long it will take before I can attempt to consolidate my debt at a much lower interest rate