It's as long as it takes.
It will depend on what's asked of you and how fast of a writer/reviser you are. Obviously every set of notes and every author will be different and even with the same author, circumstances might make it take longer at one point in their life and less time at another point.
The same editor gave me 2 sets of R&R notes. The first took a month. The second took 4 months. In true author fashion, I was worried I spent too short a time on the first set, and too long on the 2nd set
but I believe now that I spent the right amount of time/work on each. I also gave myself a 4-day retreat to boost myself through the hardest part of the 2nd set, and without that, I probably would've taken a few more months on it. I also was in a job where I had a lot of writing time on the job, so I was able to work on it more than others might. OTOH, I stressed about whether to even do the 2nd set for a month, so that added to the time it actually took, lol
I think it's a good idea to check in with her and make sure you're on the right track. It keeps your novel in her mind and also makes sure that you're not going off in a direction she didn't want (which I also have done in an R&R, lol)
