My system has been bypassed so I only go for 5 seconds max. If it just turns slowly then I let it build some speed and then push for an additional 3 seconds. Usually then it would fire right up. However, I live in Texas now and it doesn't get cold enough to justify. Plus I just rebuilt my engine and the compression is so good that it fires right up.
I had a timer power strip that I'd plug mine into. I'd only have the block heater run for about 2 hours before I went out to start it. I did that, to save power on my bill, and since the block is small it doesn't take very long to heat.
Our old powerstroke required at least 5 hours of block heater in the cold months or else you'd just wear out your batter.