When is the best time to practice Qigong?

Early morning is the best time soon after you get up.  And here's why:

After you wake up you mind is still in a low alpha frequency so its much easier to get into the proper mind state for Shibashi.
The air quality is at its best first thing in the morning so practice outside if you can and breath in all the fresh chi air you can.

According to the Bible of TCM (Traditional Chinese Medicine), its best hit the hay before 11pm in the spring and summer and at 9 pm in the autumn and winter and wake up around dawn.  Getting enough rest is something we all know we should do, but do please try to make it a habit.
...But having said that I have also found that doing Qigong between11pm and 3 am is incredibly powerful.  If you can sleep in or call in sick the next day try it out for yourself.

Everybody's body type is different.  These are just guidelines.  Most important is to practice every day and make it a habit to obtain maximum benefit.