How can I signal the scope of an adventure to players?

Question

Related: How can I close my sessions better?

I'm an inexperienced DM running my second game (playing D&D 5e both times).
The first game I ran was the Starter Set, 'Lost Mine of Phandelver', with players who had a combined total of almost 40 years RPing experience. They knew how adventures worked and could use their experience to pick up on what goals were being presented.
My current game is with 4 totally new roleplayers. I'm finding it difficult to let them know (without giving plot away) what the size of the adventure they're playing is.

For example, when running episode 1 of 'Hoard of the Dragon Queen' as a 'one night only' game, I got to the end of the episode and the players all wanted to continue straight on following the plot hooks into episode 2.
When running an introductory adventure of my own making for them (which was intended as a one-day-only game but spilled over into a second night) they picked up on and wanted to immediately follow the plot hooks at the end before we'd even spoken about whether they all wanted to continue in a regular game.

One of my players has told me she finds it useful to always have a goal in mind for whatever she's doing, and to know what the structure of her plan is. I, too, feel that my role as DM would be easier knowing there were specific parts of a story my players were progressing through - for example, so I knew when a good time to level up would be (using milestone leveling), and because all the published adventures I've read have had this structure.
So, How do I, without giving away plot in advance, let the players know what the adventure is going to consist of, so that they know when they've finished it?

Answer 1

First, let me congratulate you: you're having the problem that your players are having too much fun. As an inexperienced GM, this is the best problem you can possibly have, as long as you don't stress yourself about providing the material (don't be afraid to ask for more time).

However, the solution is easier than you seem to think: just tell them beforehand. There's no need to pretend that D&D isn't just a game; you know it and your players know it. You don't have to give away the plot just to say "Hey everyone, I'll be running a one-shot tonight." All they know is that the plot will be wrapped up by the end of the night (if things go well). There's no need to give them the false sense that their adventure might continue on if you have no desire to do so. Indeed, it's definitely worse to be disappointed that a story you liked suddenly ended than to know a story you haven't unravelled yet is going to end by tonight.

That said, please feel free to expand upon your one-shots. There's several different ways to DM, but I personally prefer to write what's happening next between sessions, not plan out the entire campaign in advance (although I will think of some good ideas on where to go next, I won't decide until later). If your players really liked one of your one-shots, there's no reason you can't make it into a real campaign, even if you never planned on it -- tons of campaigns happened this way. And you certainly don't need to write up your entire campaign as a module for it to be "valid" or even quite good.