I'm probably not the best to answer this as I'm a newbie myself, most likely more so than you. ;P But I'm going to anyway.

1) How big is your collection? 7 1/4 tsp. pigment samples, 4 lipsticks, 15 eyeshadows, 11 lipglasses, 1 blush, 3 liquidlast liners, 1 fluidline and 1 paint pot. I have this all written down in Wordpad so it was easy!
2) How long have you been collecting and what age did you start at? I started taking interest around the age of 16-17, I'm 19 now and have only been heavily obsessed and addicted for a couple months!
3) Do you give yourself a budget each time? Not really. Well, I usually go in knowing what I want and stick to that.
4) How often do you buy MAC? I'll say 1 or 2 times per week. It may not seem like it with my extra small collection, but it is steadily growing.
5) Do you sell off any of it? No. I do not buy extras to sell. Yet! But I doubt I will.
6) Have you ever lost interested or feel as though you are losing it? I'm only a beginner, so no. ;D