Sadly, the game itself does not give you any hint which completition times are required for earning gold on those respective challenges.
I don't have won this myself yet. But I can tell you a good strategy.
First step: Play and finish the challenge yourself yourself and don't mind the result. It's just that you get a feeling for it and see which parts may be troubeling you.
Second step: Observe and learn from the best. Go to any challenge and enter the leaderboards. There you can watch replays from the top ranked players by pressing X.
I don't say to try and copy each and every step they did as there are often alternatives to get the same result. BUT: Just watch and generally learn how to effectivley overcome passages you else may have trouble with. Or learn tactics you may have not been aware of before.