10 minutes on the 31 year old original arm on my K5. No heat, just plenty of penetrating oil a couple of days before and during and then a BFH hitting DOWN as directed in earlier posts. And even though my truck has always been in DRY Colorado, it must have sat in mud up to its rockers cause its more rusty that any other Colorado truck should be. (steering arm too!)
Now if you keep hitting up, you will be there all day. No matter how much heat you apply. My old Nieghbor in Michigan spent a day and a half on a 91 K5 to pull the arm off of it. (didn't have my K5 then and didn't know the trick) But it sure was fun to watch them pound on that poor arm, swear at it and almost set the truck on fire with the torch....