The old Dream Park RPG from R. Talsorian Games had an excellent adventure-development concept called a Beat Chart. I haven't played Dream Park in twenty years, but I've been writing Beat Charts for my adventures ever since. It's that useful.
In a nutshell, the Beat Chart divides the adventure into Beats, which are chunks of the adventure that either challenge or inform the players. The first Beat is always the Hook, which draws them in. Beats then alternate between Developments (plot points that move the story forward) and Cliffhangers (combats, puzzles, or other challenges), the results of which are not certain. The second-to-last Beat is always the Climax (the mother of all Cliffhangers and the culmination of the story), and the last Beat is the Resolution (the final Development that resolves everything).
"Great", you say. "What's that have to do with anything?"
Well, the important bit is that each beat is supposed to take roughly 30 minutes to play out. So, if you want to write a four-hour adventure, you need an eight-beat chart. Three of those beats are covered (Hook, Cliffhanger, Resolution) so you just need to come up with the rest.
What I've found in practice is that depending on the adventure and the group, Development Beats will probably take fifteen minutes or so (kinda hard to stretch out a Foreshadowing to a half-hour) and Cliffhangers, if they're combats, probably 30 to 45 minutes. Either way, you're covered. And, Beats also make for a logical stopping point when you've run out of time; if you end on Beat 6, just pick up from Beat 7 next week.