I had been letting my oil cooler lines soak in penetrating oil (PB Blaster) since last april. Every last one of them were still rusted solid, so last Saturday I reluctantly had to heat them to break them loose.
I always use penetrating oil anyway, but the more time goes on the less useful I think it is in breaking free rusted fasteners. Once the fastener is loosened a bit, then the oil is thin enough to get in and it helps in backing it out. But in most cases heavy corrosion seems to seal out the penetrating oil altogether, which had to be the case with these oil lines. I've noticed before, even after soaking for long periods of time that basically any internal surface not sprayed directly with the oil will be bone dry.