To way oversimplify: it's finite so you'd eventually have no water to pull out of Mars. However, once you have it and use it, you can try to capture and filter and reuse, and you'd be able to get many gallons of use out of a single cup (not all at once, just cumulatively). But if it somehow escapes, it's gonna get smacked by solar winds and you won't be able to recover those molecules, so you'd want to figure out a good setup to contain whatever water you found. It's that lack of atmosphere that'd cause all the issues, once it leaves your setup it's gone