We'd expect 40 hours a week at the advertised salary. The position is however pretty flexible because it's a small office there probably won't be a whole lot to do a lot of the time but need somebody to learn the systems and make sure problems can be fixed quickly in the event something comes up.
The only other requirement would be that the start of your work day would have to overlap a bit with the normal daytime employees (who end at 4pm generally), so I'd say you'd need to be in the office from at least 2pm during the week. A work schedule would have to be discussed.
If you're interested then please feel free to apply. Just make it clear on which hours you will be available.