I'd like to have a go at making some kind of mole, but all the recipes I have seem to make big batches. How long can I keep mole for, either in the fridge or freezer?
In the freezer, more-or-less indefinitely, depending on how good your freezer is. Certainly months to years.
In the fridge, it depends on the kind of mole. Quick moles (like various green moles) don't keep very long, maybe a week at most. A long-cooked mole should keep a bit longer -- up to 2 weeks -- since all of the ingredients are completely cooked.
If you're not planning to use it within a week, though, why not freeze it? The texture won't take any harm from freezing and thawing.