I haven't had time for my writing this week and I miss it.
I miss the way my brain wakes up when I'm working on my book. I miss thinking about my characters and how they react and think. I miss the sense of satisfaction that comes when all the words fit together and sound right.
In the past few years, probably the longest I've gone without writing is a couple of weeks. Maybe less. Last year, I even took my writing on vacation ('cause writing and having fun too was awesome). How long can you be away from your writing before you start missing it?