Highly dependent on either tank size or refugium to tank ratio. I have kept a mandarin (a dragonet) for over 2 years in a 15g then a 25g and then a 40g system that included a 25g regfugium. The long held "100g+" assessment is due to the fact that a tank that large can supply the kind of live copepods that a mandarin needs without a refugium. If you're willing to dedicate a larger ratio refugium to your setup, you can keep a mandarin in as little as 15g (I had a 25g refugium attached).
Regardless of the tank size and the size of the refugium, you will need a fully mature system. I would put a comfortable time frame of at least 9 months of up-and-running status before considering adding such a fish.