I purposely left my TX200 cocked for over a week just so that I can lessen the power a bit. Please note that this is with a stock spring. The result? No change; that is, if you can accept a loss of 2-3 FPS, which occurred for the 2-3 shots thereafter then crept back to its initial power level.
My advice is not to worry about power loss or leaving a spring gun cocked for extended periods. In my opinion, too many people get too hung up and obsessed with more power. In my earlier years of shooting airguns, I always wanted to own an air rifle with the most power. But as I acquired more experience, developed my shooting technique, and gained an understanding about the physics that govern a projectile's trajectory, etc., I learned that what really was more important was the goal of shooting with precision through acquired skills.
My Cricket PCP is set to shoot at 9 FPE with 14.3 grain pellets, and it is enough to take out pigeons at 30 yards cleanly with a heart or head shot. The bonus? Over 450 shots from 210 — 50 BAR