how long does it take to free up character slots after deleting a character and sending them to the graveyard?
As @Borogove suggested, there is an ongoing issue with the Graveyard as the already deleted character still occupies a slot. If you let me know your ingame account name and the platform you are playing on, we can remove the deleted character from our backend server and the slot will once again be available.
There was an issue where the graveyard continued to reserve a character slot. It should be fixed on PC, but I believe consoles have not gotten the fix yet. Unfortunately, I believe the plan is to wait until the end of the season.
This thread has more information on the problem, including the responses from Neocore.
I posted a workaround (delete another character, then immediately un-delete it), but you might want to make sure you save gear on the character before doing it, just in case something goes wrong.