Computer read out turnover time

I am disabled and recently was able to purchase a 2000 Olds. Bravada 6 cylinder 4 dr. With approx. 128,000 miles
I replaced the gas cap with a locked cap (which I'm sure the store gave me the wrong one because it did not fit right). The next day the search engine light came on.
Of course my inspection is up on the 30th of Nov. 2007 and the store that sold me the wrong cap RESET my computer and told me it would pass because the light was off. Well, it failed inspection because they said that the computer needed time to "roll over" in order to give them a read out.
QUESTION : How many miles do I have to drive before I can take it back. They charge me every time and I have $21.00 to my name. Can you please help me.I am a Christian and I will be happy to make a donation as soon as I get some money ---- soon, I hope. The garage told me that the car had no safety problems and it was probably the gas pressure from the wrong cap that caused the problem.I pray that's all it is. Thank you