The Valium half-life is anywhere from 20 to a whopping 200 hours, dependent entirely on the person.
The Xanax half-life is about 4-6 hours.
I wouldn't recommend long term use, as the withdrawals on both of these are among the very worst and longest withdrawals of any drug, and the withdrawals can kill you.
Also, both of these together and smoking some weed is going to cause some harsh amnesia. You probably won't remember much of anything, good feelings and all. Taking all of these together is more likely to put you to sleep, I think.
If I were you, I would stick with the weed and keep a dose of Xanax within the prescribed amount so you don't forget everything or sleep. If you were to forget all the fun or sleep, it would be a waste of the medications, right?
Less is probably more, in your situation.
Be dang sure you aren't on anything else, especially painkillers. I was just discussing someone else's possible life threatening interaction between an opiate and valium + xanax.