How much fluid does a 1992 C1500 5 speed hold?
When the lube runs out the fill hole it is full. Depends on
which Transmission you have. Manual Transmission,
NV4500] (TOPC)..........8.0 pints
NV3500]..........4.2 pints 
Approximate, fill to level of fill plug. Like Clay said
"When the lube runs out the fill hole it is full." Follow the
links and see which Trans you have. Hope This Helps.