The normal answer is “up to 24 hours”.
You can read Facebook’s documentation on this. (link)
Functionally, the more and better your ads are, the faster they get approved.
My agency ads typically get approved instantly, or within a minute.
I noticed that my ads were being approved quickly when I signed up for the “Facebook For Business” bot.
Now, I get a Messenger message for each approved (or disapproved) ad.
I try to be patient, because I know it “can” take up to 24 hours.
if my ad hasn’t been approved within a few hours (a long time for my accounts) I find a way to make a slight change and re-save/submit. This is usually something like adding a period or emoji to the text or something simple that won’t change the ad dramatically.
I find this usually accelerates the process, but keep in mind my accounts usually get approved almost instantly.
If you have a brand new account, allow 24 hours.