Quick Google search says that the average life span of a worker ant is 45-60 days. Myrmes ants should feel lucky that they get to live three months.
Not entirely, other than workers in the colony, most ants will be birthed that turn. So your nurse could birth a worker in phase two and in phase three you're already shipping them out to die. So 3 months divided by 6 phases * the two phases they are alive for comes out to a typical 30 days, give or take.
Finding out the mortality rate of the workers I'm going to have to paint little "Let 'em Work, Let 'em Live" road signs on the board.