Like you said, it varies a ton, so there is no real answer. I mean I'm here 8 months, and personally I didn't expect to see any payouts before year 1, based on my experience form blogs, it takes time to build traffic to your pages, but as it turns out I might be getting my first HubPages Adsense check in a month or two.
So while I didn't make my first $1 till the end of month 2, my traffic has increased greatly over the past two months, but you have to work for it. Making money online and from adsense is a real job, so you have to treat it like that if you want to earn. If you think this is just a quick way to get a few bucks you should stop wasting time.
So reality check, if you want to earn you have to give it time and you have to make a great effort. To go for $100 a month, it will take you a year or two, but it is far from impossible.