I had a leap procedure done in 2008 where part of my cervix was removed. And now I am 25 weeks pregnant and I was told that the baby will probably come earlier than my expected jan. 4, 2011 due date because part of my cervix was removed. I wanted to know when will it be best that I go on maternity leave. I was thinking about Dec. 17 being my last day of work.

Hi JessieNa... welcome to AMHD!

It really is going to be up to you and your doctor and the status of things at that time. While a preterm birth is a greater possibility it is not definite and each situation will have to be monitored individually.
Talk to your doctor about how far ahead of your due date he/she feels may be beneficial. The 17th sounds reasonable, but they could suggest even sooner if you were to show any indication of things progressing too early.
Enjoy the rest of your pregnancy!