Back when i first got my Nexus One and game apps were sparse on the ground i installed a free Sudoku app. I spent awhile playing it, looked up a couple strategies online, and eventually got to the point where i could usually finish the hard ones. I seem to remember them taking me about 40 minutes to do, when i didn't get totally stuck.
Eventually i moved on to other things and i don't think i've touched the app in over a year, and haven't played more than a dozen games in a couple years. But something prompted me to start poking around at it again last week.
I don't (consciously) remember the more complex strategies, but i seem to be doing a lot better at solving the puzzles semi-intuitively. (As in quickly figuring out appropriate strategies for given patterns.) After going through a half dozen or so easy ones i decided to take a stab at one of the medium ones and got it in 16 minutes. So them i decided to tackle a hard one and finished that in 20 minutes.
1: It's a very small sample size so far, and i'll start sucking again when i try some others
2: Somehow taking a couple years off allowed me to internalize some of the techniques
3: Even though i don't remember playing these particular puzzles before i may have done so and subconsciously remembered details about them (which honestly would be about as impressive as #2)