Not long until there are ad-supported relays. And that's ok. They will have to curate ads that don't suck (i.e. a relay can have Bitcoin-only ads) and maybe even PAY users to use them. It will be competitive because the switching cost is so low.

Clients could enable ad banner support that relays could leverage and share revenue with devs that way.

Nostr will be a great testing ground for advertising and users can opt out of it all if they want.