At least a month or so. I would invest in an established casino that's been around for a long time and put your money in for at least a couple of months before you consider withdrawal.
I've been already investing for almost a year in 3-4 different sites, in a month you can expect from 2% to 5% on average, having higher profit than that or a loss is not likely. This can only be applied to established casinos with big enough bankrolls to be stable. Also I started recently investing in YOLOdice, they allow up to 10x leverage on investments, which I'm using and currently finding it pretty good for relatively small investments.
it's requireda lot of patience to collect a profit from an investment like that , 2% to 5% monthly with nothing guarantee the site would live for long time enough or maybe exit scam? if you put small amount only then that investment would not worth , but when you want to put bigger amount to invest , the problem are you really want to trust that big money to a strange people? a strange site? well it's up to you.
First thing before you are planning to investing your money, you must make sure these site are reliable and trustworthy (Well, you can read other's feedback regarding these sites, check their ANN thread, or google it). I will invest my money without any doubt if the owner is e.g Baryom, Dooglus, Dean, Ryan, or Stunna. Investing your money in strange gambling sites without do any research first is a very stupid move.