I assume you'll have been given advice about not riding for a day or two then starting back gently. I'd stick to that - the consultants tend to err slightly on the conservative side but that's better than the opposite. Once the post-op swelling goes down, keep an eye on the knee - if it starts to get puffy or swollen, you're definitely doing too much.
I'd aim to get back to training for the hills over a couple of weeks otherwise you might find that you're stressing the joint too quickly. It does depend on what you've had done, though. I've had both ligament repairs (on both knees) and meniscus surgery and the latter took longer for the swelling to go down post-op but, after the former, it was quite a lot longer before the knee felt 'back to normal' in terms of being able to put some power into pedalling. But with care a lot is possible: three weeks after the meniscus surgery I climbed Mt Kenya as acclimatisation for Kilimanjaro.