I'm going to say never. The trouble isn't that the Japanese are averse to gambling-it's just that the incumbent types of legal gambling all have political pull on their side and certainly don't want the competition. The pachinko industry must have its own pull, or else it wouldn't exist. I'm not going to get into that-it's a long story why something patently illegal is tolerated.
However the horses, bicycles, boats and motorbikes are legit and have their own lobbies, and then there are the government run lotteries. Casinos would just draw the suckers from those forms of gambling and thus the government would be no farther ahead in terms of tax revenue.
Seriously, the keiren/kyotei/keiba/autobike people tend to be on the up and up both when it comes to running a clean sport, and in terms of paying the taxman his cut. Respectable people do go to horse races if not the others, and keirin is an Olympic sport. Therefore the politicians won't start competing with them with a bunch of fruit machines.