Depending on how large the screw or nut is, I melt a puddle onto it and hold the tip of the iron on it as long as i figure it takes to transfer enough heat to do some good and then apply whatever torque you can to the fitting. I used to work on tugs on the water at times and if we had seas running the skipper wouldn't allow us to open any hatches for ventilation. Since no one could be out on deck we spent time in the engine room doing projects. The boat had a Halon Fire Suppression system on it and the Captain, being an old timer didn't trust it to mind its own business because it was automatic and could be set of by several different sensors. The tow was always covered by insurance and there are pages of stipulations to follow. One was that the Halon system had to be online and functional continually and it kept a record of when it was on and when it was off and if it had been activated by an event. The insurance company got a copy of all those logs. Your score of performance was considered when asking for future quotes for coverage. He was afraid that if we used a torch of any kind it might set off the Halon so when we were battened down for seas while towing, he wouldn't let us use a torch, so we figured out ways to heat things without flame. Sorry for that long-winded story. This is an old iron I keep around just in case.
