Clivia don't need a cold spell to grow well, it just helps to get them to bloom. I grow mine at normal room temperature until they're large enough to bloom.

I agree with _dog. I grow my young clivias under optimum conditions year round until they have at least 12 leaves. This means giving them regular water, fertilizer and bright light 16 hours a day. If they are in pots, I also repot every six months or so to keep their roots healthy and growing. Using this method, I was able to get about half of my last batch of seedlings to bloom at 30 months. Usually clivias take three to five years to bloom.