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Conclusion - Unless you invest $5-$10K (thousand) to buy a good amount of ASIC's, you won't be making much money in terms of real world spending (virtual to physical asset accumulation). About 150-200 % return in 24 months.
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It depends what you consider "much money". As with any investment, the more you invest, the more you will earn (or lose).
The biggest impact on profitability is the change in difficulty. It is difficult to predict future difficulty, but I think it is fairly certain that it will be rising rapidly for the foreseeable future. If you assume that it will be rising 20% every difficulty adjustment, then your total mining revenue will be 1.126 BTC for every 1 GH/s, assuming the current difficulty is 31256961, and 99% of that will be earned in the first 6 months.
The formula for computing mining revenue (assuming constant exponential growth and constant block reward) is actually very simple:
Total Revenue = H * 1,000,000,000 * 25 * 600 * 2016 * 65535 / 2
48 / D / R
where,
H is the hash rate in GH/s
D is the difficulty when mining starts
R is the rate of increase at every difficulty adjustment (e.g. 20% = 0.2)