The common thoughts are that for every week doing nothing you need two weeks to recover fitness. This has a lot of variables contained within it though and might be a little misleading but essentially you won't gain too much by beasting yourself for two weeks before an event.
The other often quoted snippet is that it takes a few weeks for the effect of training to materialise as improved fitness. The adaptation is instant and takes time. Again, it isn't quite as simple as that but two weeks won't see you improve fitness too much if at all.
Gains in fitness made quickly also seem to be lost quickly and the flip side would be if you have been riding consistently for years then a month off might have little effect on fitness- more that you need to get your body accustomed to riding again.
IMO you are better off easing back in to the riding you would normally do in terms of the fitness/ climbing side of things and get your body used to riding again. Throw in a focus on getting the descending skills as sharp as they can be and you'll probably make up more time there than destroying yourself on a turbo so close to the event. Keeping your body as fresh as possible between now and then will also help a lot too.