I think everyone is different.
When I was working out, I would find if I didn't get back to it after 5 days, there'd be a bit of decrease at the tail end of my workout. I was huffin it.
2 weeks, much more noticeable ... I'd have mid-workout fatigue until I took about 2-3 days to build back up to peak.
3-4 weeks, was basically like cutting my max effort down by 25% and stamina down 15%.
After 4 weeks, back to square one.
And the thing that goes fastest for me, is peak strength - I've always been more of a quick, lithe, explosive kind of guy - so my peak diminishes fast if I don't keep it up. That followed by stomach and obliques. I HATE with a PASSION sit-ups.