You can apply for readmission through the UT Admissions web site. You will need to submit as a part of your application transcripts of any course work (both online and in class) that you might have completed since your time at UT, and you will need to have maintained at least a 2.5 GPA in that transfer work.
Generally speaking, students should plan on a minimum of 2 & 1/2 to 3 years to complete the undergraduate C S degree program. However, depending on how any existing university credits would apply to our degree program, a student might be able to complete their course work in a shorter time frame. Regardless, all UT degrees require a minimum of 60 in-residence (taken in a UT classroom) hours.