Nothing is 100% (board/player/etc. must be accounted for), but your bet % should be pretty high here, as you will make money long term for sure.
Personally, checking to me twice is like pushing a button: bam: I bet!
100% would be stupid because it would be way too predictable. Personally, I like to trap players who often make this automatic bet by checking a big hand to them twice. If they don't have anything, this is just about the only way to extract money from them.
My own percentage is probably around 70%. I'll rarely make the bet against more than two players though.
I will only bet if the player has not consistently check-raised and I will usually bet half the pot, if he has nothing he is folding, if he does come over the top it's an easy fold for me but I will usually take my chances to take down the pot.