For most people, I would say when the ICE kicks in, it is "hardly" noticeable.
You will likely "notice" it because I'm pretty sure you will be paying attention, and will hear the difference between EV (silent) and ICE (typical gas engine sound), plus there's a slight shudder that you'll notice if the ICE kicks in when at a standstill. The whole concept of hybrids is having a near seamless transition from EV to ICE. It doesn't really matter at what speed the ICE kicks in--they are designed to do this.
Be aware that all hybrid vehicles (including so-called "mild hybrids") have a stop/start feature that automatically shuts off the ICE when the car is stopped and restarts it when you lift your foot off the brake to hit the accelerator. Not everyone likes this, but I have had several non-plugin Priuses, so I am used to it. It's a little different with a PHEV like the A3--when in EV mode, there is no reason for the ICE to kick in until the battery depletes. The stop/start doesn't happen unless you are in a hybrid mode. I usually let the A3 etron do its thing: starts in default EV, drives until the battery depletes then automatically drops into Battery Hold mode. My 17 mile commute to the office is mostly downhill, so sometimes I manually switch to Battery Charge about half way there [based on what
@icee said above, I may have to re-think this], since less gas is needed when going downhill. This way, I still have a decent amount of battery for the return trip home. People with short commutes can get by on EV only roundtrip, not needing to put gas in the tank for months at a time. Every day I drive is a mix of EV and hybrid, so I am averaging about 54 mpg.
BTW, I had a gas A3 as a loaner from the dealer once. It has a bigger engine of course, which felt pretty good when accelerating from highway speeds, but the silent, off-the-line performance of the etron is pretty sweet. (Some might say I should just get a full-on EV, but I now have my eye on getting a used 2022-23 Q5e).
Audi USA has manuals online, but you'll need a VIN:
https://ownersmanual.audiusa.com/