an estimate from http://www.immigrationwatch.com/uscis-processin

Based on the processing records of the same type of cases in this center, the statistics analysis shows:
# Percentage of cases filed around your filing date which have been approved:
26.59%
# On average(50-50), your case may be approved around:
03/09/09
# If you are lucky(75-25), your case may be approved around:
06/22/08


As a matter of fact, for I-140 cases filed in 2007 at Texas Service Center, among all of the approved cases:
# The average waiting time at this center of your filing year is:
179 (days) » Details
# The luckiest case of your filing year at this center took waiting time:
2 (days) » WhoIs
# The least lucky case of your filing year at this center took waiting time:
458 (days) » WhoIs