Help Count Dracula complain to his doctor about his new diet. Choose the correct word to complete his sentence.

I haven't had a proper bite of O-negative blood _____ three centuries!

The correct answer is for.

We use for to talk about a duration or period of time (like "three centuries", "two weeks", or "five minutes").

Since is used for a specific starting point in time, and ago is used with the simple past to show how far back something happened.