bob- I'm not sure I understand your 2nd Q. Are you asking that if you mashin at 140, how long before you raise to 152? If so, how are you making that rise- applying heat, step mashing with boiling H2O, or decoction?
The basic Q can be more accurately answered if we know those answers. Also, what style are you brewing? At 140, you are getting more B-amylase and developing a more highly fermentable, thinnish wort. At 152, you're stimulating more A-amylase which fosters a higher % of dextrins and a less fermentable wort. Making an IPA? Keep it at 140 for longer period of time. Making a Bock? Then I'd go 15-20 minutes at 140 and 40 minutes at 152.