What is the average pitch of a plane?
I assume by "middle pitch" you're talking about the pitch during cruse? In which case, the answer is, unfortunately, "it depends". Changing pitch changes the amount of lift, and the amount of lift needed depends on the weight of the plane. So, an airplane with more passengers and cargo will need a slightly higher pitch.

Changing speed also changes lift, which will need to be compensated for by changing pitch. However, we're talking about cruise flight here, so speed will remain relatively constant.