I have heard they are bad about being outta stock. How long does it usually take them to get back in stock?
It can best be said that they are "unpredictable". Sorry, but there's no stock answer. Best advice I can give is to get on their "notify me" email list, and keep an eye out for announcements in the Ikenvape supplier's forum.
IkenVape
I don't know all the details, but Ikenvape commissions a manufacturer in China to make their cartos to their strict specifications with higher quality coils and polyfill wick material. I don't know if the manufacturer will only make so many cartos for them or what, but its not uncommon for a new shipment to become sold out within 48 hours. There's a reason why these cartos are so popular. They make better flavor and last longer than Boge or Smoktechs.
So if you really have your heart set on getting some Ikenvapes, know which ones you want ahead of time, and when you get the notification of when they are in stock, don't procastinate and buy whatever quantity you need. I'm still working on my last shipment from 6 months ago: 25 Ikenvape mega SR 510-Fusions and 25 Ikenvape mega SR Platinums. I don't notice any difference between the two models while
vaping; the Fusions have a flange while the Plats do not.