What length of time should one spend looking for a combination, and what factors should one consider? Taking time control into account, how long should one take when looking for a combination?  Also, what factors should one consider when looking for a combination?
That's a very general question that I don't think you will get a solid answer for. The chance of a powerful combination being available depends largely on the pieces each side has available and developed. So the answer varies based on what stage the game is in and the positions of each side.

Instead I would focus on identifying the types of combinations you are comfortable with, and start to learn to identify boards where those combinations MAY be present, and others where the combinations is not present. If you can speed up your identification you can determine on the fly how much time to spend on evaluating each potential move.