I am pretty sure that your goals of making your money back within 3 months are very unrealistic. Have you even spent the time doing the math to calculate your expect returns? It's the only real way to figure out just how lucrative your investment will be.
http://bitcoindifficulty.com/ - here is a site with the current and predicted next difficulty. use this as a basis. If you want to be more accurate, increase it accordingly to take into account the rise in difficultly.
Also, keep in mind that the value of bitcoins themselves at any given period is going to affect how long it takes for that investment to turn profitable. Given the volatile nature of the market right now its hard to tell if bitcoins are headed up or down right now, let alone predict their value 3 months from now.
Thank you Desensitizer, just the response I needed. Straight to the point.
I had done some calculations on current difficulty, but as you said that's going to rise. I hadn't considered that when initially looking into it. The last time I mined was in 2010 with a fairly powerful machine, but ended up in jail.
I'm going to check out that site.