Hardware RAID controller cache battery failure frequency/lifetime?

I suspect your Supermicros are broken one way or the other - possibly the battery packs are overheating. Most recent LSIs would report the temperature through MegaCLI - you might want to monitor this value on servers which needed replacement.

root@host:~/SOLARIS# ./MegaCli -AdpBbuCmd -GetBbuStatus -aALL

BBU status for Adapter: 0

BatteryType: BBU
[...]
Temperature: 41 C

I have seen a couple of Dell and Fujitsu systems with LSI BBU controllers, none of them had yearly battery pack replacement (except you screwed the pack up by deep-discharge). The typical life time has been around 3 to 5 years.