As long as it takes YOU to learn each exercise.
Sorry, that's just how it is. Any estimate of hours per exercise, can only possibly be just that - an estimate. It is difficult for you, as a student who hasn't flown before to guess how you're doing, and the only really sensible approach is to keep talking to your instructor and asking lots of questions.
That said, there are plenty of sources of the syllabus, and you can make a reasonable estimate yourself of how you're doing against the eventual 45 / 32 / 25 hour minimum, depending upon which version of the PPL you're shooting for.
But it isn't a case of learning stuff at a set rate, it's all about meeting the required standard then moving on.
G