I'm confused... do you want it to be longer or shorter than 20 seconds?
PLEASE change the warning time from 20 seconds. It is WORTHLESS.

Ok, re-reading, I guess you think 20 seconds is short.
Personally I don't start to switch into premove-mode until I'm under 10 seconds.

20 seconds should be plenty of time. By then, you should be well into the endgame, and there isn’t as much to think about. You can make over a dozen moves with that 20 seconds. Just think, some people play entire games in just 30 seconds
1. Obviously there is a desire to have a time warning of some kind, since it's a standard feature. So the only question remains as to where the warning should be useful.
2. Sometimes if you have a tough position, you're not in an endgame by the time 20 seconds comes around. You look for that one extra forcing combination and suddenly, you're way behind on time.
3. 20 seconds is worthless as a warning. A warning is supposed to tell you you're about to lose the game on time if you don't pick up the pace, not that you've all-but-guaranteed to lose the game on time. Way too often, the warning goes off and the only thing you notice is that you can simply resign.
4. If 20 seconds is "plenty of time", then why not 10 seconds? 5 seconds? 2 seconds? Heck, 2 seconds is enough to win most bullet endgames, right? That's 20 premoves after all.
5. The obvious answer is to make it configurable, but if not, having a useful warning beats the pants off having a useless one.

open dat settings then join the chess.com beta club for suggestions and mod and staff will sometimes respond

4. If 20 seconds is "plenty of time", then why not 10 seconds? 5 seconds? 2 seconds? Heck, 2 seconds is enough to win most bullet endgames, right? That's 20 premoves after all.
It depends on the position and how fast the opponent is. Outside of a fortress, 2 seconds isn't enough because you have to be ready for sacrificial moves that might generate a passed pawn (for example).
But sure, different people panic at different times, so an adjustable alert makes sense.

4. If 20 seconds is "plenty of time", then why not 10 seconds? 5 seconds? 2 seconds? Heck, 2 seconds is enough to win most bullet endgames, right? That's 20 premoves after all.
It depends on the position and how fast the opponent is. Outside of a fortress, 2 seconds isn't enough because you have to be ready for sacrificial moves that might generate a passed pawn (for example).
But sure, different people panic at different times, so an adjustable alert makes sense.
+1

4. If 20 seconds is "plenty of time", then why not 10 seconds? 5 seconds? 2 seconds? Heck, 2 seconds is enough to win most bullet endgames, right? That's 20 premoves after all.
It depends on the position and how fast the opponent is. Outside of a fortress, 2 seconds isn't enough because you have to be ready for sacrificial moves that might generate a passed pawn (for example).
But sure, different people panic at different times, so an adjustable alert makes sense.
+1
Also +1. I personally am fine with 20 seconds, but it would be nice for other people if they could adjust the warning

I don't think there is a problem with 20 seconds. Although maybe chess.com can add a setting where you can choose when you want the warning time to start.

It's so stupid to have 30 seconds abandoned game @chess.com.
I live in a country where electricity gets cut off at times, and ive lost more than 20 games just from the time I need to reconnect to the internet. That's stupid
When you hear the warning beep, it only exists to tell you that you've lost the game. It is WORTHLESS.
PLEASE make it configurable, or failing that, set it to a time that isn't utterly worthless. 20 seconds? Why not have it at two?