Common Grade Level Planning

Most schools have common planning time for teachers during Specials (Related Arts in some places of the state). Teachers have about 45 minutes to plan while students attend art, music, PE, etc. We all know that those 45 minutes quickly get eaten by using the restroom, taking and picking up students, making phone calls, grade level business meeting, receipting money and so on. So teachers end up with very little time to actually plan with one another during the school day. There is always time after school, but teachers have lives to live and can't be expected to give up many afternoons to plan together. Besides, with all the different groups that meet following school, there really is very little time to plan.
I am going to give teachers a three hour common planning time in October to try to make sure that everyone is doing things like common assessments, sharing great lessons, and planning units together. Ideally we would do this once a month, but we'll start with October and see how it goes. We did this common planning time at my previous school, but it was very expensive as we had to pay for substitutes while the teachers were out of their rooms. Luckily at my current school, I am able to use the many teacher assistants that we have to help cover the classrooms so that this common planning can occur.
The teachers were extremely grateful when they found out we would do this. I hope that they will take full advantage of specialists here at the school and allow them to help during these planning time to make sure that all teachers prepare great units for students.
I would love to hear how other schools set aside common planning time for teachers during the work day!