I seem to benefit most from fasting for about 15 hours just once or twice a week (usually at weekends). I IF'd chronically for about a week during a staycation a few months ago and I'm sure that lowered my BMR. It wasn't deliberate - I fell into my typical eating pattern at weekends. I'm a female of child-bearing age.
I enjoy having the ability to fast even though it's only for about 15 hours. Black coffee is my friend.
F 5 ft 3. HW: 196 lbs. Primal SW (May 2011): 182 lbs (42% BF)... W June '12: 160 lbs (29% BF) (UK size 12, US size 8). GW: ~24% BF - have ditched the scales til I fit into a pair of UK size 10 bootcut jeans. Currently aligning towards 'The Perfect Health Diet' having swapped some fat for potatoes.