You can use iodine to tell if you have any starch left. Just collect a sample (a drop or 2) of your wort, put it on something like a white plate so you can see color changes and add a drop of iodine next to it. Use a toothpick to mix the two. If it turns blue, there is still starch. If the iodine stays reddish, there is no starch left.
Your enzymes work over a range of temperatures so you get conversion from alpha amylase and beta amylase as it cools. Starting at 150 would get you a pretty dry beer and as the temperature goes down, the beta amylase has more effect and will make the wort more fermentable. If you start at a higher temperature the wort will be less fermentable because beta amylase is denatured at the higher temperatures. Your efficiency won't change but the fermentability of the wort will.
I'd read that the enzymes work quickly but I couldn't find a definitive number of minutes or hours so I experimented by starting a batch with the intent of taking an immediate sample and then one every 5 minutes until I got no change in color with my iodine. I BIAB so my grains were ground fine. At mash in the sample turned dark blue, so there definitely was starch. I missed the 5 minute mark and grabbed the sample at 7 minutes. No change in color, so the enzymes reacted in less than 7 minutes.
With a coarser crush, you need to get the water to the center of the particles to get the conversion since the dry grain won't convert. That's where the 60 minute mash becomes necessary as you wait for the grains to get wet and leach the sugars back out.
