VAC-Ban / Detected info

Quote from SilverFire

depends on the type of the ban.

overwatch (you get reviewed by other players @ matchmaking) -> usually about half day, up to 2 days until ban gets in.
untrusted (your cheat screws up and does something the server detects @ all valve servers) -> usually instantly, but may take up to few hours.
vac (the cheat is detected by the anticheat module @ all vac protected gamemodes) -> between 1 day, up to 2 months until ban kicks in (usually about 1-2 weeks, however may take substantially longer - usually only a few days if its a hack which has been detected already for some time, but if its a hack which is private/not widely spreaded it may take up to 2 months).