how much ram do i give postgres?

I know this is kinda a debate, but how much ram do I give postgres?
I've seen many places say around 10-15% or some say 25%....... If all
this server is doing is running postgres, why can't I give it 75%+?
Should the limit be as much as possible as long as the server doesn't
use any swap?

Any thoughts would be great, but I'd like to know why.

Thanks.

-Josh