Such a wide question..
In my humble opinion I'd say you would need to keep in mind at least 3 important things, either for the flop, turn or river:
- Your position
- How many opponents are you facing
- The type of game you have been playing on that table
Apart from that, could be a lot of implicit variables, so like I said, I don't know if there actually is a correct general answer to that wide question, more than focusing on certain points.