777 rapid decompression
Based upon several hundred walks around the outside of the B777, I would guess that the combined area of the forward and aft outflow valves would be almost exactly equal to the size of the hole in your suggested scenario. In this case, should such a hull breach occur, the system would cycle both outflow valves to the fully closed position in an attempt to maintain cabin altitude. If the outflow valve area exceeded the size of the hole, nothing would happen.If the hole exceeded the outflow valve cross sectional area, the cabin altitude would vary from the extreme of explosive decompression ( very big hole) to a cabin altitude climb at a rate determined by the size of the hole. To answer the question, if its an explosive decompression the indicated cabin condition on the MFD will rapidly show ambient aircraft altitude, plus actual aircraft rate of descent.If the hole leak rate exceeds the ability of the cabin pressurisation ( air out exceeds air in) , but at a rate below the 'explosive' level, the indicated cabin altitude will climb at a rate determined by the degree to which the fuselage breach exceeds the area of the outflow valves.
As an example, if a 777 is cruising at 37,000 ft the cabin altitude is around 6000-7000ft. Any hull breach, up to the area of the outflow valves ,can be managed by the system and cabin altitude could be maintained. A bigger hole would cause an excessive cabin leak rate, beyond system capability. The cabin will begin to climb and an immediate aircraft descent is required. From a cruise altitude of 37000ft the B777 can descend at about 6000 ft/min. This gives about 5 minutes to 10,000ft and 4 minutes to the cabin oxygen dropout at 13,500ft. Therefore, the cabin would have to climb at a rate of more than approx 1500 ft/min for the passengers to get the fright of automatic dropout oxygen deployment.
Large wide body aircraft have huge airflows through the pressurised structure and can cope with surprisingly large holes in the airframe before the decompression becomes 'explosive'.