I have personally earned around 25 million in one day, this did take about 5 hours, but realistically speaking 10 - 15 million can be earned in just a few hours.
This can also be done in any type of session (public, solo, invite only). I am also playing the '3 card poker' game but this will work for any game in the casino, I've just found 3 card poker to be the most efficient. NOTE: I am doing this in the casino penthouse, but just making sure THIS IS NOT NEEDED.
You also don't need to have any type of special building (Such as the CEO offices, biker gang buildings, bunkers, etc.) or apartment, the only thing that is needed is a casino membership (Which only cost $500). This is because this isn't a 'glitch' it's more of an exploitation of the save game system. First thing's first, yes this completely works and no it should, SHOULD, not get patched anytime soon.
