Something like a minute or two is probably the upper bound. Look at TOTP timers for a sane default (often like 30 seconds), and add a bit more for SMS delivery

Something like a minute or two is probably the upper bound. Look at TOTP timers for a sane default (often like 30 seconds), and add a bit more for SMS delivery delays. You can always try to send a new code when the old one expires. But also: basically the entire security world has been recommending against SMS 2-factor for years, because it's so incredibly easy to steal access. Don't use SMS if at all possible, don't have it as a backup (because "a backup option" == "an option" == "you are only as strong as your weakest link"), etc. Avoid it entirely.