If a Lycoming is flown on average 3-5 hours per month but kept in a hangar what level of internal corrosion is to be expected ?
Not enough to register on the "concern" scale. The 3-5 hours a month is your winning way.
"Hangaring" a plane is not insurance against corrosion, unless that hangar is climate and humidity controlled, which is rare. Fly it once a week, change the oil regularly, and don't worry about it. Remind yourself that an engine is a consumable, and one day it'll have to be repaired/replaced. Budget for that, and enjoy flying it in the mean time....