How long can my old apartment withhold my security deposit?

I live in Houston, Texas, and have been a renter for two years now. On May 10th, I moved out of my old apartment. I had a wonderful time there, and had no problems with the management company and found the staff there to be reasonable and polite. I had no problems with anything concerning them and paid my rent on time, every time, with the exception of one month where I accidentally shorted them $19. Because the amount was so small they didn't charge me a late fee and just had me write them a separate check. On the month I was to move out, I accidentally paid my rent in full instead of the pro-rated amount, and they informed me that the extra amount would be combined with my security deposit and used to repair any damages, clean the apartment, etc. then the rest would be mailed to me in a check.

It has been almost two months since then. I have repeatedly called them about it and they have said the check should arrive within 60 days. They sent a notice through the mail to my new place giving me the amount I am owed and what the rest was used for (cleaning the carpet and the like) and I was owed $423.

On the ninth of this month, it will have been 60 days. In my most recent call to them they told me they only just sent it out on the 28th of June! I'm not entirely sure I should believe them this time, since they told me they mailed it out once already.

I'd hate to have to get more aggressive about this since I enjoyed their service up till now, but I'm tired of waiting and every day it isn't here I get angrier with them. What kind of action can I take if it isn't here by the 9th?