I'm leaving for my honeymoon in a couple of weeks, so I needed a way to keep my plants alive between the times when the neighbor will stop in to water everything. So I made this watering spike.
My bit of testing showed that it took about 12 hours on average for the water in the 2L bottle to drain. It lasts a little longer if the ground is wet to begin with.
EDIT 7/26/11: Spike with Threads added. This one also includes 3 "fill layers" in the middle of the spike, which makes a sort of filter at 90% fill. It slows down the drainage a little bit, and keeps particles from being sucked up into the bottle.
Print (I used a raft and 100% fill), fill bottle with water, screw spike onto bottle, plunge into ground.
I never got around to putting threads on this, so it's a little weird getting the spike on the bottle. You may want to print out the first inch or so and stop the print to make sure it will screw onto your bottle. If not, you could scale it up a little and press in a bottle cap with a hole cut in it. Someday, soon, I'll actually make threads.