Do you mean if I am sedentary for a while?
I recently was in an accident and could not do much of anything for about a month, and then another 6 weeks until I started back to my usual exercise, which is mainly zumba, and some swimming. In the two months time I think I lost more stamina than muscle strength. I noticed it especially when I swam, because I usually would do 20–25 lengths, and the first time back in the pool at 10 I was done.
As far as the muscle, I have muscle trouble anyway so it is harder to gauge. I was very careful when I first started back to zumba and doing things around the house, I was on a restriction for how much weight I could lift, so I didn’t really get to test how week or strong I was, and so I worked my way back slowly anyway. I have found now and in the past muscle strength comes back quickly. Within a few weeks if you have had some loss with lack of activity you can build back up. Unless you are a body builder or very extreme in your exercise.
I remember hearing on some show talking about weight loss how much fat and muscle is lost when people diet on average. I would guess there a scientific studies on musce loss when simply not working out or being bed bound, etc. Hopefully someone will have more scientific information.