In any case, the MATZ only 'exists' when the station is open for operational flying, but as welkyboy says, the atz is in operation (usually) 24/7. However beware; many military airfields host flying when the station is not operational and ATC are not on duty; this can be light aircraft, winch launched gliding or SLMG.
It's generally accepted that if you make 3 calls on the notified frequency with no reply, the MATZ is not active. If you then try 122.1 (NATO common and you MUST mention the station you're calling), it's possible you might get a reply as some military airfields use this as an A/G frequency when ATC is not operating.