They can back track 5 hours, but it is difficult. The blood will not be returned for several weeks, perhaps months. It is important that you select and begin working with an experienced DWI attorney, because this is a very serious situation.
Tracking back or retrograde extrapolation requires a lot of information in order to be done properly. Five hours is a long time and will put you in the elimination stage. The single most important piece of information is the results of that blood test. Regardless of the blood test, you are in a very serious situation. When you factor in your prior criminal history, it makes this situation even more serious. I recommend talking with an attorney experienced with felony DWI defense as soon as possible.
You are facing a year or more in jail. You should retain a good Avvo DWI lawyer in your area and hie an expert to try to show that you were not intoxicated or that the blood test is not accurate.
Hire a lawyer immediately. It is not clear if you refused to provide the blood initially, but you may still have time to request an administrative hearing to contest any license suspension. If so, your lawyer can subpoena the officer to testify under oath, prior to trial; this can be a great benefit to your defense. Generally, the state needs to know the time of your last drink and other facts to calculate what your Blood Alcohol Content was at the time of driving (it is against the law to be over .08 when driving but not against the law to be over .08 when tested). You need an experienced DWI lawyer ASAP. This is a very technical area and you have a lot to lose.
I agree with my colleagues, you need to hire an experience criminal defense attorney as soon as possible. Lab results for blood can take a very long time. Most crime labs are pretty backed up and a month is not an unusual amount of time to have to wait. Retrograde extrapolation or "back tracking" as you called it is dependent on numerous factors. However, the State is not simply bound by your blood alcohol level. There are three ways to prove intoxication in Texas 1. The loss of the normal use of your physical faculties 2. The loss of the normal use of your mental faculties, or 3. A BAC of .08 or higher. In order to determine if they can prove intoxication at the time you were driving the State will review all evidence (dash camera video, witness statements, driving facts, and the blood alcohol level just to name a few). Based upon your criminal history and the stakes involved, you need to retain the services of an experienced criminal defense attorney as soon as possible. If you would like, you can visit the State Bar of Texas Lawyer Referral Program at www.texasbar.com in order to assist you in your search for an attorney.