I started pinholing with the M8 I had on loan and an Australian made Raytex pinhole adapter for Leica thread mount. With the M8 I found that 7-15 secs was adequate for open shade to dull conditions at 160 ISO.
I also found that long exposures chew up batteries! And leave a lot of 'noise spots on the image.
Later I'll use the Raytex (available on ebay) on my Epson R-D1s.
I am most pleased with the portraits - it's not hard to get people to hold a pose for 10 seconds!
Even if they move, the results can be pleasing.