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.