just means 'considering', thinking about something, maybe weighing up what to do. It will vary according to the circumstances. You should ponder what car to buy a lot longer than you ponder whether to put tomato sauce on your chips or not.
'Ponder' comes from the same Latin source as our word 'pound', meaning a measure of weight. So, as J suggests above, it is a matter of balancing all the factors involved in making a decision. Clearly, asking how long should be spent on the process is rather like asking 'How long is a piece of string?'