What you spend your time doing is who you are. It is everything that matters about you. Deciding how to spend your time is quite the challenge.
A good requirement for the purpose of your actions would be to ensure that what you spend your time doing improves your life or that of any part of the cosmos at all (deciding whether something is an improvement is a whole other discussion). Another good requirement to decide if you should dedicate much of your valuable time towards something is to ask yourself if you will be one of the best at such a thing. I can think of two basic heuristics I use to decide if some habit is worth my effort: if many people agree that I am excellent at whatever I am doing and if I can spend many hours in a row for days in a row doing this certain thing. It is also always a good sign if you become above average at something within a short amount of time. One problem with these requirements is if you are awesome at many things. This may cause confusion. Really though, only one set of actions will be the absolute best in regards to overall benefit (probably).