probably a few months to get it back, depending on what mall section you manage to get. Malling is apparently one of the easiest ways to make a ton of np. I never managed to upgrade my shop, but I know plenty of people who have and never regretted it.
malling is just being a part of a main market mall. they usually have very overpriced items, but it sells anyway since people who are going to the main market are either unaware of the shop wiz, on a faerie quest, or just casual players who want to buy a cute item for a few thousand np.