There is no ROI guarantee, doesn't matter how long the time frame, the cost of running equipment will rapidly outpace anything it can earn in btc. Reward will drop by half approx. every four years, next due around Aug/Oct 2016, hash rate will sky rocket, electricity costs will rise significantly. You'll be trapped in a loop of pouring more and more fiat or earned btc money into ever more efficient miners just to stand still. Anyone not upgrading in line with the increases will just end up being squeezed out of the process.
If you're going to start mining for profit now you're insane, for a hobby fine - no problem, otherwise just buy btc. All the benefits of increasing btc "value" with none of the overheads, stress and hassle.