so I keep it cocked and locked. How long can I keep it in that condition without damaging springs?
Essentially indefinitely.
But the springs are not what you need to be concerned about. What you need to worry about is the LUBRICANT.
Over enough time, many oils will turn to sludge. And that's a bad thing.
When my father passed, we found his Colt Govt Model in his dresser drawer. Loaded mag, cocked and locked and ready to run. I doubt he had touched the gun in 10 years. Possibly more than twice that.
after removing the mag, I cleared the gun, then let the slide run home on the empty chamber. The slide slooowly went forward, and then stopped, about half way shut. The oil in the gun had thickened over the years, and essentially turned to glue.
The gun was detail stripped and fully cleaned, and once the old oil sludge was removed, functioned flawlessly. I did NOT unload the mag (GI 7 rnd, fully loaded), and a couple months later, when we did get to fire it, it ran perfectly.
Its NOT the springs you need to worry about.
I would point out that if it doesn't rust, a 1911A1 will normally run through at least a full magazine with no lubrication in it at all.
If you live somewhere you need to protect the gun from rust, consider a good wax or silicone coating.