Hi
I don't HAVE to leave it but do do on occasions. If the property is empty and no one is going to be in there then it can be the easiest option. Just pour in, go back next day and flush, time taken is 5 mins or less, no elbow grease, no scraping ( so no damage to the toilet)
Most of the time I will clean the toilet and then pour in the acid and then just leave it while I clean the rest of the property and flush before I leave, sometimes I need a loo brush, sometimes I dont.
Depends on time, situation and how bad it is.
Recently did a toilet that 2 other cleaners could not get rid of the limescale, charged £30 for 5 mins work which has now led onto over £300 of work.
Use what you feel comfortable with, as long as it works that's fine.
Regards
Martin