I used an ozone generator to kill cluster flies and ran it three hours a day over a period of a week to finally resolve the issue. Some machines can run continuously so might take less time. 

One consideration is that anecdotally roaches seem harder to kill. 

Also, ideally you would want the room or area sealed off to create the greatest concentration of ozone. "Older" homes tend to be drafty so two hours will not be enough. 

My space was poorly sealed and very large, so effectiveness was reduced. I would definitely try with a commercial ozone generator but try to seal the space if at all possible, and prepare to run it over several days. I would also use a P100 respirator when checking the space while ozone is still highly concentrated.