30 seconds is added on as a rule of thumb, not a rule of the game. Plenty of google-able links to confirm. Yes if someone sprints on and off it may be less if the referee doesn't go to default. Equally somone walking off from the corner flag risks more being added. But as a standard rule of...
OP - 30 seconds is added on and teams do it so regularly that a ban has been suggested.
https://www.independent.co.uk/sport/football/news-and-comment/football-lawmakers-ban-injurytime-substitutions-premier-league-a8602481.html
No, the clock is not stopped. It's not rugby. It keeps ticking and the ref has to add an approximation on.
CH could have brought me on and we'd still have won. Daftest thread today.