You may or may not need to flush the old fuel out. Yes, fuel does go bad but how fast it goes bad often depends on how and where the car was stored. As primitive as this sounds, if the fuel in the tank "smells funny" or not right, then it probably isn't and it should be flushed. If necessary, siphon the bulk of the fuel out. You will get all but perhaps a quart out of the bottom of the tank. Then add about 3 gallons of fresh fuel to dilute what old fuel was left behind. At the carburetor, disconnect the rubber hose and aim it into a catch bucket. Remove the spark plugs and the main lead from the distributor so the engine won't fire and so there will be no sparks. Then have an assistant crank the engine to pump the old fuel out of the lines. Once you move a couple of cups of fuel you are done. Connect everything back up.
You asked about how long to crank to develop oil pressure. If you have a gauge... as soon as you see the gauge climb off zero. If you have a warning light, crank until the warning light goes off. If oil pressure doesn't build, post back and we'll walk you through how to re-prime the oil pump.