It's upto you mate, just see how you feel from it, I am NOT someone that wants to spoil or stop your fun mate.
You noticed how quick they kick in too?
Your tolerance also shoots through the roof when you sniff 'em, just do what you feel inside yourself.
Just watch it for a loss of feeling in your hand/fingers & really dizzy feeling upon standing up, I sadly cannot tell you how much to do as in my experience some people can snort several & be ok, then I have seen someone snort half of a 7.5mg & he was sick as a dog, I knew he was gonna be fucked up really quick as less time it took to boil the kettle for a cuppa tea he was having those visual b&w flashes, 20 mins gone by his p.
IMHO I'd give it 10 mins between each one myself.
Welcome to the weird ROA club, you understand now why I use this ROA?