[Lazy]

Actually, no.
Its nowhere close to that.

Make a float variable, lets say flFinishDuckTime...
When you want them to hold down the key set it to gpGlobals->time + 5.0 or whatever time measurement you want.

Where the code for holding down buttons normally goes add something like this...

if gpGlobals->time is below flFinishDuckTime then hold down the IN_DUCK key.

I think that's how it goes, though I still wonder if you thought the line you posted was actually valid.