There's nothing to stop you from leaving a plastic snap cap chambered after dropping the hammer. In general, both mainsprings and magazine springs can take a set over time, losing strength, but how much time depends on the spring design and how compressed it is. Also, obviously, it depends how much extra strength margin was designed into the spring.
I've never had either the mainspring or magazine springs in my 1911 take a set, despite spending extended periods of time cocked and locked. My old Ruger Mk I bull barrel pistol had a magazine spring that got weak and had to be replaced. I got that gun used for $68 in 1968, and the spring only went south about 5 years ago, so maybe that gives you some idea of the time required. I've probably put 25,000 rounds through it, and have no idea what went through it before I got it. At one point in the middle of its service life for me, I had to chamfer the back of the bolt tunnel because it had finally battered enough to drag on the bolt. So it's seen some wear. I normally only put 5 rounds in those magazines at a time, but it's like 6 because of my extended followers. The followers consume one round of capacity, so they make the magazines rest with the equivalent spring pressure of one round being in them all the time.
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