I'm a bit of a gamer, L4D happens to be one of my most often played titles. However, if you have Team Fortress 2 then there will be more free downloadable content coming out either today or tomorrow... the Spy update!
On Survivor mode I have either gold or silver for everything, but there are a few maps I'm 5 seconds to 25 seconds away from the gold but never quite managed to get it! I don't know what my longest time is, I think 12-14 minutes was it not counting a few times I was testing map exploits. Unfortunately there seem to be quite a few of those people are beginning to use.
You really need a cohesive, good team to make gold, that's for sure! And lots of luck, random chance is everything. The hospital elevator level is probably the easiest for living the longest.