Yes, around 10%-20% time for design looks correct to me - which is why you have maybe 1-2 designers in a team of 10 people. But the designer still has to work 100% to get stuff done and it can get very stressful as well. And his work is important - because design-errors are early errors which means they are the most expensive ones you can have.

But it's really as in all jobs - the difference between good and bad designers is so absolutely unbelievable huge that if you worked with some bad ones and then meet good ones you hardly believe they are doing even similar jobs.