100% and i use that percentage rarely.
If you edit
src/server/scripts/EasternKingdoms/zone_undercity.cpp
only that file will be recompiled.
if you edit
src/server/game/Entities/Unit/Unit.cpp
everything will be recompiled.
unless you do full rebuild, then everything will be recompiled.
also 3 hours recompile? what processor you have?