According to what Outhue wrote, you will need at least 440 days of non-stop mining just to break even. And thats assuming current difficulty and BTC price.  Mining difficulty always goes up, and BTCs price swings wildly.  Your electricity cost is practically free, and its definitely the biggest advantage you have going for you. But apart from the cost of electricity, buying mining hardware is also a big hurdle. You really have to think about a bunch of stuff. Like, what if your machine just kicks the bucket before its even paid for itself? These arent just regular electronics; they are basically running full-throttle, 24/7, pushed to their absolute limits. Its not unusual for something to break before its time.