Like you said everyone is different. I have found that depending on when I ate last or how much liquid I consume can vary the intensity or length.

Can I ask, do you have no tolerance because you don't regularly use cannabis or are new to it? Do you know how much smoking a few bowls or a joint(of quality cannabis) would effect you?

Generally if your getting your edibles from a dispensary the tender will give you advice on dosage. However, the link below is a really good read on edibles in general and they have some dosage info for people with average tolerance.

Cheeba Chews | Product and THC Medicated Industry Information | Medical Marijuana Edibles

Personally, I have a coconut oil recipe where a 1/2 cup of oil in a bread loaf recipe that will send me to space on 2 slices. But I have not yet had a great experience from "dispensary" Activated THC MG rated edibles, I had a 300mg Snockers on an empty stomach and felt the effects but not nearly the effect I like from that size edible, I have also had some 150mg truffles that did a better job than the snockers, and I have tried a 70mg chew a few times that had a VERY mild effect(they were sativa tho, still need to try the indica and hybrid versions)

But that is just me, and I would say I have an extremely high tolerance to THC in general and edibles in-particular as I am pretty big, 6'7" 200lbs, And I use flowers and shatter throughout the day.

So I guess the only real advice I can give you is, start off small and see what happens.