The first generation of Tonmo bimacs that I kept records on had three bimacs that lasted 10 months or more. Since they were wild caught,they tended to be a bit older when they arrived, too. My Ollie was 3-4 months old when she came (my estimate) and lived about 10 months, so that would make her 13-14 months old when she died.
No one here on Tonmo has been able to keep a bimac in a tank for much more than 10 months and this corresponds to the experiences of other bimac keepers before Tonmo.
I don't know where the "1 1/2 years" comes from - can't seen to track it down.
Nancy