I dont know, but it seems like it's very often.
Lets see:
94: diadora (this is as far back as i can remember the kit brands)
95: nike
96: nike
97: nike
98: nike
99: kappa
00: kappa
01: kappa
02: kappa
03: puma
04: puma
seems to me it's every 4 years. After each world cup, they...