Let's say I can invest 0.20BTC for 6 months, is there an average on what I would be earning or even a potential, thanks. 
No one can say how much you can earn in that timeframe as your profit will depend on how much the house wins against the players, if the house loses ofcourse you will lose too so there is no guarantee
There should be a semi predictable amount of return if you know the daily betting volume. I am not sure what it is, but Quickseller put up a formula a few pages ago that shows you how to calculate it.
No its not. It's called variance and happens in the short term. If you know daily betting volume and the houses edge, you can easily calculate what your expected profit should be. In the long run you will be right on, in the short run, you'll show variance. It's called math and it's absolute.
Please scroll through dicesites.com and look at how spot on the calculations are in 2 years time.
Even moneypot got An expected value of 477btc but got a Profit of 697btc, and this is when we just had a big loss of almost 60 btc (went from 430 to 370)....
That is called short term variance. In the long run, as I have said, you will always end up at an expected profit. Its just math and you can't argue with that. 2 years time can be considered short term if your bet volume is low enough. I don't remember enough statistics to show how long you would have to look for based on a given volume to be within a single confidence interval, but it def can be done and calculated.
Which makes it kinda look like you just want to make a theoretical math Point that can't be used for the questions People ask most of the time.