Waxing often isnt going to save your string
It might even add to a quicker death. I dont wax my strings unless they get rained on... Anyways, I would say a year if you shoot often, and or if you expose them to the weather alot (i.e, rain, heat, etc). BlackStick one of the guys that used to frequent this forum a few months back said he had around 10k shots on his VT's and they were still not bad... Every set of strings is going to be different per individual use, so i would just go with a year or two at the most.
Derek