Well, maybe never. Maybe tomorrow. First, they need to be mature enough. Yours may not be yet -- if they were tank raised, tey could be less than a year old. If they were wild caught, they could be that young or as old as 30 years.
They need good food and lots of it, and very stable water conditions. A nice warm steady temp (80-82F) and long light cycle helps.
Some fish are more sensitive to light changes and disturbances than others. My pair will (and have!) spawned while I was cleaning the tank. Some pairs can't even have the light timer get slightly off or it will throw them off schedule.
So that's where to start looking at your husbandry -- consistancy and nice fat fish. If you have already satisfied all these conditions, you can possibly jump start a pair by extending the lighting period and simulating a plankton bloom, but you may just need a bit more patience.