I have seldom spent more than a couple of minutes planning any flight, even IFR. With experience, it just doesn't take that long. A glance at the map gives approximate distance and heading, and you should be able to calculate the time required in your head from that. With current GPS units, it's easy to crank up, take off, and navigate to a destination with little or no prior planning. You really need to know the fuel burn for the aircraft you're flying, and be able to figure the required fuel for almost any distance, with reserves, close enough, without even needing a calculator. Long offshore IFR trips can require more detailed planning, especially at the limits of fuel range, but even those should only take a very few minutes with experience. Getting the experience, of course, does take time, and long involved planning for the first flights, until you really know what you're doing.