I remember many years ago watching a City v Southampton match at Maine Road, and using the stopwatch function on my watch to record the actual time played. I stopped it when the ball went out of play, or a foul was given, etc, and restarted it when the ball was played. The actual playing time was 21 minutes, 30 seconds! I didn't bother with the second half.
I still look at the clock today, and the average time for a goal kick is 25-30 seconds, and 15-20 seconds for a throw-in. That is, of course, if the team we're playing ISN'T time-wasting, when it can be much, much, longer - eg, Tim Howard.
As for the point that the match could go on for 2-3 hours, I don't think so. Once the players, managers, etc, realised that 90 minutes football WOULD be played, there would be no point in time wasting, feigning injury, as they would be late getting to their wives, partners, mistresses, prossies, as it was their own time they were wasting. We, as fans, would actually see 90 minutes football.