The average bachelors-to-doctors degree time is about 7 years. The are some officious rules trying to constrain this. Some are negotiable. It's important to talk to the grad office before you hit any of these limits.
The Texas state legislature put State Law SB 961 (“the 99 hour rule”) into effect in the fall of 1999 in an effort to encourage Ph.D. students to graduate more quickly. The rule states that after 99 doctoral credit hours, graduate students cannot receive in-state tuition. Out-of-state tuition is two- to three-times higher than in-state. Furthermore, this rule applies even if you are a Texas resident.
To fully understand this rule, you must understand what is meant by “doctoral credit hour.” If a student comes to UT for a Ph.D., the student is not automatically classified as doctoral on arrival. If the student has a baccalaureate degree then he or she will initially be classified as being at the Master's level, even though he or she may not be interested in getting a Master's degree on the way to the Ph.D. The student must accumulate at least 30 hours of study before becoming eligible to be classified as a doctoral student. A student who wants to get a Master's degree on the way to a Ph.D. can of course do so, and for the first 30 hours would be classified as a Master's student, but thereafter could be reclassified as a doctoral student even if the Master's degree wasn't finished yet. For students entering with Master's degrees, the habit has always been to try to determine whether the degree was germane to the doctoral degree the student intended to get. For example, a person with a Master's degree in English, but entering the Physics department, would not be considered as being more advanced than someone entering with a baccalaureate degree.
The bottom line is that Ph.D. students entering UTCS with a BA or BS degree must complete their doctoral degree in 130 credit hours, which translates into about six or seven years.
Graduate students may only be employed by the university (as a TA, RA, AI, etc) for fourteen long semesters (fall and spring). Since this amounts to seven years, it coincides fairly well with the 99 hour rule. There are other details about graduate student employment that you can find here.
While there is no official time limit on acquiring a doctoral degree, there are a number of limitations on work that applies towards the degree. At the time of admission to candidacy, all completed work must have been done within the previous six years. Once in candidacy, students are encouraged to finish within three years.