What is a good way to tie off a campaign when i think it has gone for too long in a way that will be satisfying for the player 

boss fight

Thank you for that

There was an adventure in 2e that was specifically designed for this purpose called The Apocalypse Stone. May be worth a read. The end of the adventure has the multiverse essentially come apart at the seams. If the characters can manage to save the world, which is very difficult in this adventure, the world is at the very least permanently transformed, paving the way for a new campaign.

Another option is to go the way of 4e. Have every character pick an epic destiny and have them work to attain some form of immortality, ending on a high note.

Aim for a TPK, if they survive, even better send off. If not, their death can be the inciting incident of the next campaign. 

I'd suggest ending on an accomplishment which feels difficult but the players succeed. Follow it up with a short description (with input from the players) on how the character's life proceeds after this adventure ends.

However, I'd prefer to create one last plot line which might take a few sessions that would bring the campaign to a satisfying close - after which the characters could retire. It could end in a boss fight as suggested above or have the characters save the world or at least the local country. (e.g. stop a marauding dragon, save the kingdom, retire as heroes).

Make sure that you have let the players know the campaign is ending - you don't want to just say to them at the end of the session "Well, that's it, no session next week since the campaign is now done." or "Ok, that campaign is done, create new characters for next week.". Either approach is likely to cause some problems. Ending a campaign is something that you should talk with the players about.

I'd also a avoid a TPK - unless the players are ok with it. Players get invested in characters, killing them off and then saying the campaign is over so that there is no way to get them back stands a good chance of creating angst and bad feelings.

That's kind of hard to say without knowing more about your specific game, but:

In general, it's good to end on a big event, tying up whatever big unresolved plot has been an ongoing thread. Since you're asking, I suspect you don't have one of those, and the game's been more episodic.

The first option is just to say to the players "You know, I think it's time to wrap this game up. Is there any big, unfinished business that you want to deal with?" 

If there isn't, you can just wrap, or go digging in their past adventures for some enemy who can be powered up (possibly brought back from the dead, or as undead) make a suitable final conflict, or some element to put under threat, like a city they're all connected to, or a favorite NPC who needs their aid.

Another option is the "getting the band back together" plot: Do a time skip. If they weren't already max level, give them more levels and a few new toys. Let them tell you how they've been spending their twenty/forty/a hundred (sometimes they're all dwarves and elves) years of relative peace. Get them to tell you about their families/students/other investments in the future.

Then... dire portents. Ominous omens. A Threat is coming, and great heroes are needed to stop it. Ideally, this threat is linked to something from their past, but new can work, too. It doesn't have to, and probably shouldn't, be a long, complex story. Big direct fight/hold out until the ritual is complete/hunt the lost artifact/gather allies from around the world are good tropes for this.

There's lots of room for fun roleplaying. And, as long as you don't make it stupid or pointless, character death is a plus here. (Have some backup NPCs to loan out in case death comes before the final confrontation.) They're probably going to be itching to do a heroic sacrifice. Staves of the Magi will be shattered.

And, if you want to run another game in the same world, you have a shaken-up status quo and low-level characters with a built-in connection. (If everyone had families, everyone playing somebody else's kid could be fun.)

Edit to add: If you make the threat serious but not world-ending, you can be open to the possibility that they will lose, in which case the follow-on game can be really different. Don't force the loss without player buy-in, but be open to it, and be clear to the players that it's a thing that can happen. (With the right players "Preserve what little can be saved in the face of the oncoming darkness" is a fun trope.)

Introduce the Deck of Many Things. It's like a built-in epilogue machine. 

You can also get this out of the players via Role Play.  Put them in a tavern with a bard who wants to hear their stories, get them thinking about all they've done and then ask "so... what's left for you?"  Take a few of those and set up a final push/ adventure/ boss fight to just that.  Maybe even it's a case of a God who wanted to give them one last epic moment of their choosing before helping them ascend to the pantheon, or a dragon who can see that these adventurers are going past their prime and wants to give them an epic send off into retirement.

Something else to consider in this - Is your campaign world ending ( ie are you going to create a whole new world for the next campaign) or continuing after the characters “retire”?  This can have a serious impact on how you end the campaign. 

Start a new campaign, and on the first act, have the new characters explore a ruin, and find the remains of their old adventuring party, with lots of clues as to why they were killed and by who, causing them to swear revenge against the BBEG who did it.

Then have them work towards some form of time travel

Then have them go back to assist themselves, and do an epic bossfight with each player controlling 2 characters.

Then have a paradox rift open, and have them choose which character to sacrifice to bring normality - they can only have one!