If your into solo mining it will take you for about 98 days, crazy right? So it isn't advisable at all. Try pool mining or upgrade to ASIC-based mining hardware. I've read that you can make $0.000485032112 per min, or $0.02910192672 per hour or ~$0.69 per day. How cool is that?
Disregard this information... He just made it up. 98 days ROI with 20 - 30 Mhash/s? Not a chance. It'll take much more than theoretical years to earn anything more than a dollar and his calculation of earnings is ridiculous too. Earnings are never set in stone, they fluctuate depending on many factors (pool success at solving blocks, payout changes, volatility of crypto prices, your own hash rate essentially serves as your shares which determine the dividends, the ever rising mining difficulty will have drastic effects on diminishing returns.
If you plan on doing this OP, make sure you do it for learning purposes only as R2P stated above. For folks going to universities, if you're dorming, you generally don't pay for electricity so this is an ideal place to get some free wattage.
Few things you'll want to note when starting to look for mining hardware.
Make sure it's not Nvidia. AMD cards are well known to have much higher hash rates than their Nvidia equivalents. Of course ASICs dominate the market in this day and age, but that requires significant monetary investment.
CPU mining, just don't at this point. Not even worth it for testing as they are considered fossils of the Bitcoin mining era. It will eventually shorten the life of it due to the high strains the hash calculations put on it through constant use, thus making him lose even more money if he does this.
If you ever get serious enough to want to mine and don't have the funds necessary to do it yourself, try and find legitimate group buys on various forums that pre-order the latest and greatest Bitcoin ASICs and split the profits (some massive miners are going to be released this year, TerraMiner IV from Cointerra and Neptune from KnC).
To give you a perspective on just how powerful these miners are, the Neptune, which will be running at 3 Thash/s is equivalent to approximately 100,000 Nvidia 9800 GT's.