Hi, in my opinion “spell failed” is dictated by you. If you really think that the spell will fail because you put out the candle too soon, it will fail. But imagine that you have a candle that is supposed to burn for 20 hours (like mine…) and you need to go to work in 12 hours. Do you really think that the best option is to leave the candle unsupervised? I would blow it out, maybe put it out with my fingers, and close the ritual properly, thank the spirit that I have summoned to help me etc.
So don’t worry, I think its okay and it will work for you, just give it a time and don’t lust for outcome.
Here is another topic for you that you should consider reading if this spell was for you and your ex to return them to you: