I just received a revise and resubmit from an agent. I've been querying this book for what feels like forever, and was just about to trunk it because no one seemed interested. But now I have this agent wanting me to make some pretty substantial changes that I think may take me quite a while to execute.
She hasn't given me any timeframe in which to get back to her, so now I'm wondering if I need to drop everything and focus on this revision so I can it back to her ASAP. Or is that not the expectation? How long can I take over this?
First off, congratulations on the R & R! That's a fantastic step. It means the agent has connected with your story, sees its potential and now wants to know if you can revise and make it shine. Hopefully she has also given you a few helpful notes to guide you as you do the revision. I also hope those notes align to the vision you have for the story. I've seen R & Rs that writers have really struggled to get through because what the agent wanted the book to be didn't align with the author's vision of it.
So, if you are happy with the notes and prepared to do the work, you need to take as long as you need to in order to make the book the best it can be. An agent isn't going to be impressed if you address only the specific things she mentioned in her message to you and send it back in two weeks.
She wants you to really take on board what she is saying and apply those things to the entire book. And you've mentioned that the changes she's suggested are substantial. Maybe changing a POV or amalgamating two side characters into one. Maybe one of the storylines is underserved and needs to be pushed up in the narrative. I don't know what the changes are, but it's important you do them well.
These things all take time, not just to write, but to think through. You want to get this right, so you need to take time over it. But you also don't want so much time to go by that the agent forgets she even requested you send it back. So my advice is, take as long as you need to make the book as good as you can. But don't waste time either. If they're a good agent and really believe you can make the story shine, they are not going to mind waiting six months or so for something polished. I've even heard of authors being signed after holding on to an R & R for a year.
But in reality, an R & R is a test. You're being asked to show how well you can revise because this is important once you get into the editorial phase of publishing. You want to be thorough, but you need to be able to turn things around to meet deadlines.
So really consider the notes she's sent. Thank her for the feedback and let her know she can expect a new draft in X weeks/months. Think realistically about how long you think it will take, how much time you have available to work on it, and then add two or three weeks to that number so you have a buffer in case life throws you a curve ball.
Now get to work! You have a revision to do.