It all comes down to how much time you are willing to accept between paydays and how much variance. Remember bitcoin mining is a poisson distribution (

http://stattrek.com/online-calculator/poisson.aspx ). Lets assume you are not willing to go one difficulty period without finding a block.

If you have 1/2016th (0.05%) of the network you should find on average 1 block per difficulty period.

Expected = 1 block

Probability of 0 blocks = 36.7%

Probability of 1 bock = 26.4%

Probability of 2+ blocks = 36.7%

You have a ~1 in 3 chance of doubling (or more) your return but also a ~1 in 3 chance of earning nothing. That is probably more risk than most miners are willing to take.

If you have 2/2016th of the network (~0.1%) you should find on average 2 blocks per difficulty period.

Expected = 2 blocks

Probability of 0 blocks = 13.5%

Probability of 1 block = 27.1%

Probability of 2+ blocks = 59.4%

Looking better but lets look at 5 times as much hashrate, 10/2016th of the network (0.25%).

If you have 10/2016th of the network (~0.5%) you should find on average 10 blocks per difficulty period.

Probability of 0 blocks = ~0% (1 in 22,026)

Probability of 1 to 5 blocks = 6.7%

Probability of 6 to 9 blocks = 39.1%

Probability of 10+ blocks = 54.2%

So less than a 7% chance of earning less than half of expected. How about a nice even 1% of the network.

If you have 20/2016th of the network (~1%) you should find on average 20 blocks per difficulty period.

Probability of <6 blocks = ~0% (1 in 13,906)

Probability of 7 to 13 blocks = 6.6%

Probability of 14 to 19 blocks = 45.9%

Probability of 20+ blocks = 52.9%

Now we are looking good. Only 7% chance of earning less than 70% of expected. Of course your risk tolerance will vary but the same distribution can be done at any percentage of the network. As the difficulty growth goes down you may not necessarily care about how many blocks in each difficulty period but rather how many blocks per month since the powerbill is due each month. There are ~4,320 blocks per month which means the probability of extremely bad or good luck over that longer period of time decreases (quite a bit).