Making canna-brownies tonight!

Well, I've been smoking the good tree for a while now, but I've never made weed brownies. So tonight, myself and some friends are going to make them. We only want to use an 1/8 of cannabis. I'm using this recipe: http://applesoft.com/brownies/ . It seems pretty thorough, and it corresponds nicely with the brownie mix that I bought. The recipe suggests 5 grams and 1/2 cup cooking oil, so I think if we use 3.5 grams with the 1/3 cup of cooking oil called for in my brownie mix.

Any recommendations? How are the effects? What should we expect for onset (1 hour?), and duration (3 hours?)?

Thanks much!