When is flight planning done?
Anytime convenient! Honestly. You can plan for a trip 3 hours later, tomorrow, next week, or even next month. There're already pre-defined airways on most areas you'll fly above; you just need to lookup the charts and select yourself a route.
Now, unlike a family road trip, commercial flights are flown everyday. Every pilot assigned to the flight will make that trip. So, no, pilots do not arrive at the gate, being told "Your next hop is going to Hawaii", then grab the charts, lay them out on a nice big table, and figure out which waypoints to go. Somebody at the company has already done that for them. That somebody is called a dispatcher.
And remember, it's the same route flown over and over again, day by day. You only have to plan once.
When do I know how much fuel to take?
To figure that out, you need to know how much cargo and how many passengers are going on this flight. When will you know that? A few hours before departure, you already have a good estimate (given that usually x% of passengers will turn up). By the time check-in for this flight is closed, you have the exact number. Just plug in that number into a calculator and you'll get your fuel requirement. Even without computers, this calculation takes only a minute.
The pilots (in particular the captain) has the final and unquestionable authority over any matter of the flight, including its flight plan. That said, pilots are not the ones who come up with the initial plan. In airline operations, the pilots' responsibility is to adjust the plan as necessary. For example, if bad weather is expected at the destination, the captain might request extra contingency fuel.
So in short, the pilots' responsibility include:
- Familiar themselves with the planned route
- Review the weather forecast and NOTAMs, and adjust the flight plan if necessary
- Verify the weight and balance is appropriate (taking into consideration cargo, fuel and passengers. Again, somebody has already done the calculation for them.)
That's not much, indeed.