That depends a lot on the company. I've seen game-designers which took it easy, but I've also worked with good ones which took their job extremely serious.
The good ones are able to catch problems before stuff is coded or modeled. A good designer manages to describe every object needed (including rough sketches what they should look like and describing needed animations), every mission condition (which is very close to coding and needs good understanding what is technically possible). A good designer can describe the GUI pretty much ahead in a way that coders don't have to rework it over and over until it fits (including describing stuff like left-click/right-click/drag&drop/selection box behavior which can get very tricky pretty fast in games). A really good designer might even know excel good enough to calculate all kind of stuff through before even approaching any coders.
A bad designer thinks it's all just about some great idea and throwing in lots of stuff and the team will do the rest. A good designer knows it's 90% about describing details and about removing as much stuff as possible.
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.
The good ones are able to catch problems before stuff is coded or modeled. A good designer manages to describe every object needed (including rough sketches what they should look like and describing needed animations), every mission condition (which is very close to coding and needs good understanding what is technically possible). A good designer can describe the GUI pretty much ahead in a way that coders don't have to rework it over and over until it fits (including describing stuff like left-click/right-click/drag&drop/selection box behavior which can get very tricky pretty fast in games). A really good designer might even know excel good enough to calculate all kind of stuff through before even approaching any coders.
A bad designer thinks it's all just about some great idea and throwing in lots of stuff and the team will do the rest. A good designer knows it's 90% about describing details and about removing as much stuff as possible.