I started 2 weeks ago or so and so far haven't found 2 out of 9 I actively looked for. One of the 2 was while we were in Osnabruck and I think it was geared more towards those that knew the area already and whatnot. Plus my mother was still foofooing it (I converted her) and didn't want to hang around in that area all day.
Of the 9, there were also 2 that took going back to several times. One was a micro (found that sucker tonight! first micro find!), had to go back 2x. The other was actually our second cache and a multi that we accidentally started. Around here they say Ikea tests marriages...no, I think multi caches do! ;-) But, after trying 6x over 2 weeks, we finally found it last night. When we finally got the final location right (we were there 3 of the 6x) we spend probably about 4 hours total looking. We found it when we were about ready to do another walk of shame back to the car.
And incidentally, I found out that the area of the forest I've been jogging through for the past 3 years has a randomly spawned set of concrete stairs leading up to a giant fire pit. That was a little surreal.