THAT is the real question, isn't it.
Not very goddamn much as your skills go up.
Critical Fail: 1 out of 10 is an autofail or roll-again situation. That means ballpark 2 auto-fails per session if a busy skill-session and not heavy RP instead. I also tend to not have rolls unless under stress, so that's a factor. You aren't going to crit fail opening your car door, as the corebook points out.
You can spend Luck to avoid this, of course, but even so, I use a second die roll 50/50 chance to see if it's an auto fail, so about 1/20.
One auto-failure in 20, before the player spends Luck, seems to work out for us. That's auto failures. You can still roll and have to add your skills in and fail from not being good enough of course.
Skill based failures it really depends on the skill. In the 2020 system, Skill Level 5 is generally competent. 6 is Professional ( 6: actually the skill level a programmer or dancer, for example, is described as being professional).
So if you have average stats of 5 and you are competent, you will automatically succeed at Easy tasks ( providing you don't roll that 1/20 critical fail) and about half the time, without gear or preparation you will succeed at Average tasks, just "winging it". With some gear and some prep time, you ( Made Up Average Stats Competent Skill guy) will automatically succeed at Average tasks and have a 50% chance with Difficult tasks on your first try.
As you might guess, if you are talented, like above-average stats and cyberenhanced with good gear, well, Difficult tasks become Easy pretty fast.
So a skilled expert with good gear should hard fail, under pressure, before spending his Luck, maaaybe 1/20 times on Difficult tasks. If the pressure isn't too great, if he's unwounded and undistracted, if he has good gear and some prep time.
In the real world, that expert, if for example he knows it's a dangerous or critical task, spends his Luck and makes sure he doesn't auto fail. So, he can only auto fail when he runs out of Luck. Less than 1/100 times.