I can solve Monday's and Tuesday's, usually under ten minutes, and Wednesday's about half the time. Once I solved a Sunday crossword (roughly equivalent to Thursday), with four other people, and it took us three days.
Never solved a Thursday or Friday. The Saturday puzzle is known as the "B word-mother of all crossword puzzles" (from the Times Crossword puzzle editor himself). I've never gotten more than five words in that.
I actually use them as diagnostic tools. If I can't solve a Monday puzzle, it's a sign of clinical depression.