For me it's about the third or fourth day. It's not so much speed as it is stamina and coordination. I can get the coord back pretty quick with certain warmup exercises, the stamina part is more athletic and might take a few sessions. And there's nothing like steady gigging to keep one in shape.
I kinda live by this rule,
"If you miss one day of practice, no one notices"
If you miss two days, you notice.
If you miss three days, your friends notice.
If you miss four or more days, EVERYONE notices!"
I went a 1 1/2 weeks back around Xmas. I had to give my hands a rest. They always hurt and ice just about everyday. The rest was awesome and I practiced mentally a bit. I was very worried that I'd be so rusty that my hands would forget how to even play a C chord! When I picked it up, I had some nice ideas. My hands were right where I left off and no different than if I just skipped a day.
When you take about a week off, you often find that your playing is better than you thought. When you play everyday you are so close to the playing that you often do not hear results and growth. After taking some time off you will hear how far you have come. Now after about 2 or 3 weeks I notice the slop starting to creep in.
This was my thought upon viewing the title of the thread.
I've been working on my basement studio so I have been focussed on construction which cuts into my normal practice time. At one point, I went a 2 week period without having the guitar in my hands for more than 15 minutes. (yeah, just a handful of quickies really...) But in the last two evenings I have had space to sit and focus for a bit.
I did notice my dexterity was not there at first, so I decided to *muscle* through it. I put the metronome on and started working through excercizes. I hit my wall of trying to go as fast as I know I can through them and backed down on the temp several times and focussed on just doing the right technique.
After two nights of 1/2 hour stints with that I feel like I am back to my normal feeling when I play to a few backing tracks. When the room gets put together and we can jam on a regular basis, I'll be much happier.
I used to be afraid of taking a break, but now I don't worry so much.