Apparently there's a forumer on H!O who's sorta a "statistics compiler" and listed the tenures of MM leaders. The 6 longest are:
1. Takahashi Ai was leader for 4 years, 3 months, 29 days
2. Iida Kaori was leader for 3 years, 9 months, 15 days
3. Nakazawa Yuko was leader for 3 years, 7 months, 9 days
4. Fukumua Mizuki is leader for 2 years, 6 months, 9 days
5. Michishige Sayumi was leader for 2 years, 6 months, 8 days
6. Yoshizawa Hitomi was leader for at 2 years, 22 days
So currently Mizuki has the 4th longest leadership in all MM history