My understanding of spring engineering (which is not professional grade, but not none, and I have successfully made working springs) is that a new spring takes that "set" very early on in it's operational life. For mag springs that's like 10-15 cycles. Then it just holds that [new] uncompressed length basically forever, or until it gets enough use cycles that it looses it's elasticity and is "worn out".
It is possible you can get that "initial set" from loading once and leaving compressed as well as the "break in" cycling that normally happens. Properly engineered springs take this into account, for example if you replace the coil springs on your car you are supposed to let them "settle" for a couple days or a short drive before setting final ride height, as it will change a little.