When to change timing belt on ford focus

hi i bought a ford focus recently and the person that i bought the car of said that the timing belt only needs to be done every 100thousand miles and said it was done at a 100thousand. i have recently checked the timing cover and cant seem to find the mark that says it was done. there is now 124thousand miles on the clock. should i be worried and how often does it need to be replaced??? any solutions would be greatly appreciated