^^^ They don't know how long would it take yet. The different MOXIE runs have different targets. The first was a technology demonstration that you could carry out the catalytic decomposition in Mars environment at reasonable current (heat) and you could make oxygen. They've done just that, which is a feather in their hat. Now they have to step it up to much higher current where the production becomes more efficient; if this is ever used, it will be in that regime. But MOXIE is very power hungry, keeping it at 600-800C even in the Martian day, even with the aerogel insulation, takes power. And keep in mind that the rover has an RTG that produces only ~100W of power, a light bulb level... MOXIE may never reach its full potential because of insufficient power (it's actually likely), but the oxygen production should scale with power in a certain way, and they will try to extend the range as much as possible.
