It is way too easy to nail a zombie for at least 15 seconds of flame time. I know how it works and I'm still saying it needs to be reduced. I can't count the number of times I've been set on fire for 35 seconds today and yes I do try to avoid the damn nades.
Easy solution, at least on a ZM map. Bind !ztele to a key and as soon as you hear a nade hit that key and presto, no fire.
Yeah unless you're playing a LOTR map or deathstar escape; then you either eat it or die from the afk spawn killer.