The updates is rather small, takes less than five minutes on a 512/512 ADSL

- Peter

While you can't turn off updates to the steam engine (which are always
tiny anyway - the Steam platform itself is pretty small), you can turn
off automatic updates to any or all of the games (Note that if a game is
not 100% up to date then you must let it update before you play it).

this might be helpful if steam keeps updating a game you rarely play
while you want to play something else.

but as Peter said, they're usually not that big. (Then again I guess if
you start out from scratch after having not updated in months or even
years then it might be a pretty large update..)