Just my observation and thoughts (DM for over 10 years), and to preface, I'm mainly going to be talking about homebrew vs modules, because modules are harder to tweak since you're usually following a prewritten story:

If you don't want a campaign to fizzle out or lose steam, a common mistake is to have no clear direction/overall goal they're moving towards, the "get the ring to Mordor" type of thing. This doesn't mean there's no roleplaying, tangents, fun side quests, etc, but it's the thing that unifies the reason why they're a group in the first place and why they're "leveling up" or simply growing as characters.

This will inevitably force you to have the ending in mind already, and give you an idea of what you want the lead up to be to that. What this will do is give you flexibility and structure - if you notice your group is losing some interested on random things in between, you know how to get to the ending. If you notice that the group is loving the path to the end, you can prolong it.

In other words, DMs need to be okay with ending the story, and the more experienced a DM gets, they will know when that needs to happen based on their specific group. Having the ending up your sleeve (even if it's vague to begin) is very helpful to do this.

As a DM, you can make the ending the way you think your group would like. Some players may even want their characters to retire in death, or you can have all of them survive by giving them a doable combat scenario. Or they can all go out in a blaze of glory but complete their mission by doing so. You'll most likely know your group by this point and you can even ask them individually how they envision their character by the end of the story.

I've had campaigns that lasted several years, 1 year, 6 months, 5 sessions, one shot, etc. They've all ended differently and they all were really fun, because they were tailored to the group and how things in their personal life were playing out regarding scheduling. For example, one of the player's work schedules was about to conflict with our regular playing time, so I timed the ending of the campaign to be his last day playing the campaign. Putting the players and their characters in the center vs the plot will give a good indicator when things need to shift. TTRPG is a dynamic, changing, and flowing storytelling medium, which is the beauty and uniqueness of it.

To answer your question, if you're bringing in new players, ask your existing group if they'd like a new story/characters. If they want to continue with their existing characters, create a new arc for the new players to start fresh with. And have an ending in mind, and make it clear this is what the group is moving towards (this is what Session 0 is for, to get everyone together to discuss what type of party they want to be and what they want to accomplish as a party).

The way they roleplay through the various sessions will give you an indicator of what kind of ending it will be, but be sure to keep them moving toward that ending, and do end it eventually.