"Security updates guaranteed for: nn months"
or
"Security updates guaranteed until: YYYY/MM"
There should be a also scheme where companies who claim these things would have to deposit some calculated amount in some offical place.
(Btw, since so there's so much cheap crap being imported from china, I guess the most consumer-protecting approach would be to make both the reseller and producer responsible for the penalties resulting from a lack of security updates.)