One of the basic concepts with mining is 'difficulty' - this is an adjusting factor set by the algorithm every 2 weeks to ensure that an even number of bitcoins are mined every 2 weeks irrespective of how much computing power exists in the ecosystem.
This creates an arms race where more and more compute power is deployed until it becomes uneconomical except at scale or with new generation chip designs. (sidepoint - 22nm is the current generation of chips commonly available from Intel etc except certain BTC ASICs are shipping with 20nm chips - to Simon's point of 'doing something more useful' :)
below is an example of how difficulty can bite the miner hard. After 10 weeks at 15% difficulty a 1TH miner will have earned over 50% of what it is ever going to earn, ever. After 6 months it is at 83.5% etc. Diminshing returns and the cost of electricity will determine when you turn it off.
Difficulty | 15% | ||||
GH | 2 week period | GH to earn 0.1BTC per day | BTC earned | Running Total BTC | % of total |
1000 | 1 | 7000.00 | 0.20 | 0.20 | 13.4% |
1000 | 2 | 8050.00 | 0.17 | 0.37 | 25.0% |
1000 | 3 | 9257.50 | 0.15 | 0.53 | 35.2% |
1000 | 4 | 10646.13 | 0.13 | 0.66 | 44.0% |
1000 | 5 | 12243.04 | 0.11 | 0.77 | 51.6% |
1000 | 6 | 14079.50 | 0.10 | 0.87 | 58.3% |
1000 | 7 | 16191.43 | 0.09 | 0.96 | 64.1% |
1000 | 8 | 18620.14 | 0.08 | 1.03 | 69.1% |
1000 | 9 | 21413.16 | 0.07 | 1.10 | 73.5% |
1000 | 10 | 24625.13 | 0.06 | 1.15 | 77.3% |
1000 | 11 | 28318.90 | 0.05 | 1.20 | 80.6% |
1000 | 12 | 32566.74 | 0.04 | 1.25 | 83.5% |
1000 | 13 | 37451.75 | 0.04 | 1.28 | 86.0% |
1000 | 14 | 43069.51 | 0.03 | 1.32 | 88.2% |
1000 | 15 | 49529.94 | 0.03 | 1.34 | 90.1% |
1000 | 16 | 56959.43 | 0.02 | 1.37 | 91.7% |
1000 | 17 | 65503.35 | 0.02 | 1.39 | 93.2% |
1000 | 18 | 75328.85 | 0.02 | 1.41 | 94.4% |
1000 | 19 | 86628.18 | 0.02 | 1.43 | 95.5% |
1000 | 20 | 99622.40 | 0.01 | 1.44 | 96.4% |
1000 | 21 | 114565.76 | 0.01 | 1.45 | 97.3% |
1000 | 22 | 131750.63 | 0.01 | 1.46 | 98.0% |
1000 | 23 | 151513.22 | 0.01 | 1.47 | 98.6% |
1000 | 24 | 174240.20 | 0.01 | 1.48 | 99.1% |
1000 | 25 | 200376.23 | 0.01 | 1.49 | 99.6% |
1000 | 26 | 230432.67 | 0.01 | 1.49 | 100.0% |