THIS is the difficult thing to remember. I have EV electric rates at home that start at 11PM. It frustrates me when I forget to activate the timer, plug in, and then the e-tron charges immediately because I forgot to activate the timer (wish you could just have it automatically activated everyday per a schedule like my Pacifica PHEV -- why you have to manually turn this on EVERY SINGLE DAY is not helpful). So now I've adjusted my routine to plug in, and stand by the charge port for 15 seconds until I confirm the green light fast flashes instead of slow pulses, indicating the timer charge vs. immediate charge.
read what i post again, (this works on the GT, not sure if it's the same for non GT (but i assume it's the same))
the trick is to have 2 timer enabled, this allow you to only need to activate timer mode once, until you manually deactivate from the mmi again.
here is my setup
Home location, Time to charge 9:00PM to 4:00PM (my Time of Use rate for lower rate)
auto release AC charger when done charging is "enable" (this allows me to unplug without unlocking the car, and not accidentally unlock and lock the car again without unplug, and it jump to next timer and try to charge again)
timer 1 Monday, tuesday, wendsday, thursday, friday, departure 7:30AM,
timer 2 Saturday, Sunday, Departure 9:30AM,
make sure they are both enabled, in the app or in the car,
now you only need to turn off the car and activate timer mode one time from the car.
and it will now not deactivate timer charging mode, when done charging, unless you manually do so.
this makes it whenever i plug-in on the weekday, it will only charge after 9

m (car flashes blue timer mode) and when it finish a charge at 7:30AM, it won't deactivate timer, as timer 2 is active and pending.
and whenever i plug-in on the weekend, again it won't charge until after 9 and when it finish a charge in the morning, it can't deactivate as Timer 1 is still active and pending.
(only bug i notice with this, is on friday, it will say pending charge complete 7:30AM on saturday, and on sunday it will say pending charge 9:30AM on monday, if i check the app before 12:01AM as it hasn't crossed over to the next day, once it cross over into saturday and monday, it will correct itself, and show the correct pending charge departure time.)