From what I have read/heard, there is no set time limit on when the feds will require states to convert to mile-based exits. Right now the states with sequential exits include NJ (turnpike only), CT, NY, VT, and NH. CT and NY have started converting a handful of highways but the bulk will take several more years. VT has done a "soft conversion" (adding placards saying "Milepoint Exit #" but retaining the sequential numbers (for now). NH hasn't even started, and a bill was introduced to convert but was shot down. Hmmm.... odd... where have we seen this before in NH? (cough... commuter rail...cough). Eventually the feds will crack down and say you have until 20## to convert, or you'll miss out on the 80/20 funding.
Still it seems weird to see MA and ME converted, while the 16-mile stretch of I-95 along NH's seacoast retains sequential exits. And Sununu's argument about "I was an Exit ## kid" I just don't buy. But, again, it's NH, so live free or die, I guess.