I just checked my USCIS application dashboard for my I485 filing. It says: "Effective as of: January 31, 2014: Texas Service Center processing time - 5.9 Month(s)"
5.9 monts, I assume, is an average. As Amazon have mentioned, for some processing might take 6 months or more. For some (including myself) the process took just a little bit over 3 months. I saw cases being approved in as little as 2 and even 1 month on trackitt.com. With this being said, your time might come sooner than you expected. Good luck!