I've heard some people say around 8 months before they are fully housebroken... but I guess it varies. Tyson is 4 months and he can hold it all night on the weekends when I get to sleep in, but on a normal day when I get up at 3 AM to go to work, he's ready to go out.
Have you tried taking his water up a couple hours before he goes to bed? That could help.
If he's consistently going at night every night, you need to be taking him out at that time. Having accidents is going to delay him being 100% housebroken.
If this is something that started happening all of a sudden, you could always take him to the vet to rule out a UTI.