If I have the supplies, I test once a week. If I start an antibiotic, or something that may change my INR, I may test a bit more often.
Recent experience with my TWO InRatio monitors (Alere replaced my possibly defective monitor) suggests that the difference from lab values could be uncomfortably high -- perhaps as much as .5 or.6. This means that if my InRatio tells me I have a 2.8 (theoretically in range), and my INR from a lab is closer to 2.0, I may have to be a bit concerned.
I don't have an XS, so I can't say anything about correlation with blood draws.
I just got cuvettes for my ProTime 3, and I'll be using that - weekly, probably - to compare to my InRatio.
At this point, with a possible difference of 1.0 or more from one meter to the next, I'm not convinced of the accuracy of either meter (or is the lab messing up, too?).
For me, weekly testing makes a lot of sense.
Back in the days when you had to drive to a lab, wait to be poked, wait for the results, pay the lab and the doctor, etc., doing monthly tests made some sense. Today, for five or six bucks (okay, maybe a bit more), being able to do the test at home makes it seem pretty reasonable to test weekly. At least it does for me.