You've launched a large project at work, something that's been in progress and taken up large chunks of your life for more than 6 months. The post-launch triage is over. Tech support isn't calling you every hour because they don't know how to troubleshoot an issue. Your hours drop from 60+/wk to whatever is normal in your organization (which is hopefully less than 60+!).
How much time do you (or your team) need before the next large project begins?
I was asked this question at work and I think the ideal minimum is two weeks -- one week to clear your desk and inbox + one week to clear your head and remember what it's like to have a life outside of work.
I'd frankly acknowledge that just being asked this question is a huge boon to work/life balance. But I do think it's possible to go too long in between.