I have a normal insurance from my workplace (it is like all other corporate insurances). Am I exempt from Québec Prescription Drug Insurance Plan?

Yes.

In the "Quebec Prescription Drug Insurance Plan" section (Provincial > Quebec Prescription Drug Insurance Plan), select Box 14 and if you had private company insurance for the whole year, "This covers my situation". 

What if the coverage was for the half of the year? How we can mark it in the system?

If you contributed to a group insurance plan through your company for PART of the year, you would select:

1. "This does not cover my situation"
2. answer YES to "You were covered by basic prescription drug insurance provided by a group insurance plan of which you were a member, or your spouse, mother or father was a member, and this plan covered the cost of medications." and click Continue
3. select the months where you were COVERED by the group insurance plan.