It depends. The best racers traing 7 hours plus. I spend about 2 a day, and I race weekly. (and win sometimes too!) It really depends on what level you want to race at. If you just want to enjoy the sport 30 to 90 minutes is fine, it just depends on what you want. If you want to race, an hour a day is a bare minimum.
It is actually possible to be a very competitive cyclist on as little as one hour of training per day, but the training must be focused and specific. Know what your objectives are before you go out and ride. Just riding a lot of miles won't necessarily make you faster. My brother and I were both Cat.1 racers. The most miles he ever trained in one week was 350. On the other hand I never rode less then 500 miles a week, but I tended to waste a lot of miles. For me anything under 100 miles was a short day. Nowadays I know the wisdom of training better not longer.