Which should I choose and why:
e.g. There is/are 5 minutes left.
Well I think that when we want to refer to individual minutes the 'are' choice is better and when we refer to a period of time the 'is' choice will do, but I am not sure about this.
You're right, and there are other threads on here dealing with exactly that fact. In your particular example, although I would say an argument could be made for both of them, I much prefer "There are five minutes left". However, if someone said "There's five minutes left" (especially if they use the "there's" contraction) I would not challenge it or suggest that it was wrong. I simply prefer the sound of "There are" with such a small amount of time as five minutes.
What about if we say "a few hours of Photoshop work" the verb should be singular or plural?
e.g. "A few hours of Photoshop work are/is hard work."