Just a quick campaign

I've found that almost every one-shot I played in took 2-3 sessions.
I've managed to run 100% of my one-shots to time though, Here are some tips that can help with that.
Always schedule an encounter before the final battle that's an optional scene, take it out if you're over-running.
Make sure to have NPCs around who can hurry the characters along if they're dawdling too much.
Write plotlines that contain an element that motivates the characters to solve them soon (e.g. kidnapping, escaping villains, deadlines for results.)
Make regular sacrifices to Chronos, the god of time.
Try to run scenarios where the characters start already knowing each other, so introductions can be sorted out in summary during the introduction, rather than at the table during play.
Release a swarm of ravenous goats into the room 15 minutes before the session is supposed to end.
Make sure your that your one-shot is planned to take about half the time that your group could spend at the table, so you can over-run a little without trouble.
Try to keep table-talk to a minimum, request players put their phones away before you start.
Plan to have food after the one-shot finishes, so people start getting hungry and push the game along.
- Shoot the slowest / most disruptive player in front of the others after about 1/3 of the time available has elapsed. This will help the others focus.
  
  Btw, bullet points can created by beginning the line with * or -.
- The food thing seems like the real winner here.
  
  Especially if they have to compete with the goats for the food.

My group's last one shot took five sessions lol
- I’m planning one and pitching it as a 3 shot
  Combat always takes longer than I think, and I know they’re going to grill to whatever random merchant is in the town for 2 hours instead of talking to the mayor who requested their help.

Mines of Phandelver was a year-long thing for us
- I'm starting this as our first dive into DnD later this week. Did your group completely go off the path of the main story? I've read through and it looks like a 4 maybe 5 session story to me.
  
  It's at least 7 sessions unless you're constantly dropping hints. Parties are stupid, I found the map in the goblin fort but I don't think we ever looked at it.
  To be fair we did have two goblins tied to ropes that we were using to check for traps.
  
  It developed a lot, for example had a reoccurring character from the Goblin encounter called Dibb, he'd always escape. Every session just added more and more lore and eventually ended with them having their own HQ in the abandoned village where they faced the Dragon.
- Same
  It was a very fun time though!