Technically, the mission will respawn until 100% atmosphere is reached.
But with actual configuration, almost anyone won't do that. It takes too much time. You need 40 magnetic shields missions to reach 0 atmosphere losses with high atmospheric values without magnetic generators.
I modded my game to make magnetic shields double powerful, also the missions respawn quickly up to three missions at the same time.
But it's a private mod that I never released (mixed with a lot of terraforming adjustment to my taste).
If you are interested in a mod like that, you can tell me and I could rebuild it and released in paradoxplaza.