I can 110% tell you for a fact and several thousands of miles. Several years. Unbelievable amount of heat cycles that strictly grey permatex works on its own with zero front or rear gaskets. Been 10 years since I've used one and 20+ engines.
Glad it worked for you, but no engine would EVER leave my shop with the intake sealed with permatex sealer. Now on the china walls, yes. But the OP is talking about doing away with intake gaskets between head and manifolds. That's a NO from me