That depends on many things. Age, size, how much it eats, temp, stress, etc. While they are young you can expect it about ever 1-2 weeks. As they get older that slows down.
2-4 times a month is normal throughout their life-cycle. I've had them shed often like that too and when their young it's nice because their colors brighten faster.. If it continues to be to rapid that it causes you to worry I would start looking at your setup and maybe adjusting the humidity and cool and warm sides.
Is it common to not see them shed? It's kind of the same question. I've seen changes in her spots & coloring, but never once caught her in active shed. FYI: I'm only 5 weeks into being a gecko owner.