There are plenty of ways to create time-lapse photos and videos, but most of them are taken over the course of several hours. If you want to do a longer term shoot over several weeks or months, you'll need a battery that can last that long, and you probably don't want to leave your DSLR sitting somewhere for that amount of time anyway.
Instructables user fpound wanted a camera that could run without him being near it for up to 4 months, so he built this long term time-lapse rig to capture an entire season for a project for Kontent Films.
It's essentially a camera with a zoom lens, a voltage converter, and a timer remote control housed inside a Pelican case.
It's connected to a car battery to make sure there's enough power to last a couple of months. Depending on your duration, you'll also want a media card with a pretty decent amount of storage. The mount is a quick release plate that attaches to the case via two L-brackets and some PVC pipe.
The process requires some electrical skills, but nothing too crazy. Just make sure you either know what you're doing, or read up before you start, because 12 volts may not kill you, but it probably won't leave your camera in very good shape. Here's the basic diagram.
Check out the video to see the setup and results.
For the full step-by-step process and more photos, head on over to fpound's tutorial.