As you suggest, it is very hard to define. For the past 15 years, I've been writing and running con events to fit a fairly strict four hour time frame (give or take).
I now try to keep scenario text to 10-12 pages. That's 8.5x11 inch paper with a half-inch margin all around in a font approximately equivalent to 12 point Arial. Note that a bigger group takes more time to shepherd through the same story. My scenarios are usually written for eight characters, though most can run with four.
Keep in mind that NPC
interactions, action scenes, and such can make a great difference. Likewise, any point at which you follow the characters in an unexpected direction can throw the allotted time out of whack. You can find yourself constantly varying your pace to try to keep within your bounds.
A handout can be a huge time sink if it must be read by each player. Party splits require delicate juggling as you cut back and forth between scenes. I always recommend isolating the investigators to reduce red herrings and wild goose chases, at least until you are comfortable with managing improvisations.
Of course, you'll be dealing with a home group, so it's always possible to say "Continued next time" if the story runs long this week. Always be prepared to give your players room for good roleplay when they get into something. That is, after all, the name of the game.