As I've heard, the newest xrite color-meters tend to drift after the one-year mark, but significantly less than their older models due to the newer ones having a more sealed path. If you expose it to a lot of temperature/humidity changes by field use, it will drift faster. If your long sessions somehow cause the unit to heat up significantly, that can also hasten its gradual drift a little.
I believe it might be possible to have it recalibrated/offset measured at a dedicated camera shop. You'd have to call around, but the worst they can do is tell you "no". That's indeed sad to hear that it's so prohibitively expensive to simply send it in to the company for a tune.
If I'm getting models confused and you're asking about a spectro, then I've yet to learn much about their drifting possibilities aside from them being significantly more stable over time.
Reading up on colormeter drift is the one thing that's kept me from jumping in on the fun; so if an inexpensive fix pops up, color me interested!