Not a bad animation.. If you notice though, it's one simple movement forward in each step. So, moreless this took someone a long time, but it isn't all that complex to build.
Check this guy out, multi animations in the same graphics moving at different speeds. Really good work and a mixture of things to look at, really showing off his skill.
Example:
http://www.mahjqa.com/sigs/dnamahjqa.gif
http://www.mahjqa.com/sigs/index.html
http://www.mahjqa.com/sigs/index1.html
http://www.mahjqa.com/sigs/index2.html