Reasonable number of person-months for a full time academic? I am currently applying for an Italian grant (a PRIN, precisely, in case someone is familiar with them), and in the application we have to indicate a number of person-months also for the PI and the other participating academics that are already employed full-time by their own universities. 

What is a reasonable number of person-months to include in this case? How much of his/her time can a professor/academic (so someone who already has a full time teaching and research job in a university) contribute to a big research grant?

There is no explicit guideline in the call.
If you are working full time on the grant (e.g., as a post doc or research scientist with no teaching or administrative duties), then 100% of your time is not unreasonable. If you have a standard faculty position with a breakdown of maybe 40% research, 40% teaching, and 20% admin, then you might be able to get away with 40% of your time, but most people would expect you to have other, possibly "unpaid" research commitments. In my field, funding agencies generally do not like to see less than 10% time (half a day a week) from the PI, because that just does not leave enough time to administer the grant. For young investigators, they want to see commitments of at least 20%.