They will breed when the female is ready, though I have found red humps do best in a harem of at least 3 females to 1 male. A harem situation can save the female from being killed if she is not quite ready, so the male can focus on a female that may be more ready than another.
Some large water changes can sometimes work as a trigger, mimicking the rainy season. I will often do a water large change everyday when spawning appears to eminent.