Reasonable number of person-months for a full time academic?
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%.