Most USSD menus have a similar time out, and yes 20 seconds is more than enough, the pin is limited to 5 numbers so it's easy for any phone use be it a touch screen, qwerty keyboard or feature phone be able get it done

TheBTCManual

bitcoin

How To Transfer Bitcoin Via USSD Code

> [...] you can send the satoshis to either other Machankura via their phone number or a Lightning Address.
> 
> - You would need to access the USSD menu
> - Select send bitcoin
> - Enter the number of sats you want to send
> - Enter the phone number or lightning address
> - Input your pin, and you’re done.

In the Twitter spaces conversation it was stated that the USSD command would time out in 20 seconds.  Is that true?   Is 20 seconds enough time to enter phone number and PIN?  (nonetheless an alphanumeric Lightning Address ?) 