See the Corona discussion in the Politics thread for an example. I'd think a max of 1 or 2 levels of quotes is sufficient. Can't imagine anyone would object, but maybe I'm wrong.
There's a discussion here:
https://www.phpbb.com/community/viewtopic.php?t=2493276
f you mean you don't want people to be able to nest quotes then you can set that value in the admin panel > post settings
Maximum nesting depth for quotes: 1