I got into CrO2 after working with hones for a while. The number of shaves between touchups depends on several variables:
1) Edge properties (hardness, bevel angle, how fine you honed it)
2) How often you use it (if you have 10 razors in your rotation, 15 shaves per razor is close to 6 months)
3) How tough/dense your stubble is
4) What you expect from the razor