I will (and have) spent hours in FMs, though it is sometimes split up over a couple/few days. I seldom start playing an FM if I don't have at least about an hour to devote to it. I often have a notoriously slow playstyle, so even 'quick' missions can take me a decent chunk of time, but I'm seldom disappointed by 'time taken'. If it was short, but the story/gameplay/architecture/etc. was engrossing, I finish quite pleased. Too long is only too long if it was dragged out/not engrossing. Using DarkLoader, I left notes on my completed FMs. I believe out of all the FMs I've left notes on (and I've played the vast majority of them), less than a dozen even mention time, and only a couple of those would be disparaging, so it's not really one of the aspects I rate an FM on. An FM takes the time it takes, and so long as the time spent was enjoyable, I'll take as little or as much as the FM author deigns to create for!