You would essentially be seeing the same ROI that the site gets in that time. So if the site has a bank roll of 100 btc, and you had 1 btc invested. On a site wide profit of 100 btc, they basically jumped their investment by a 100%, so you would do the same and would get only 2btc in total, which is the initial percentage you had invested in the bankroll.
What do you mean "only" double your investment is hardly "ownly"
I dont think that their is any fixed return in gambling sites as if some players play bingo game and hit the jackpot and just run away then that loss will be in investment also, so i think you can not get assured income in the gambling / dice sites, it may go profit or loss also.
There can never be a fixed return other then the % of profits allocated investors. You will lose money but in the long term you will make money. If the site is making profits so will you.