The following article has some samples which are close to what you want to do
https://community.landesk.com/support/docs/DOC-26389
Personally I'd have a Hours and Minutes int16 attributes on a note and total those up to a decimal Hours attribute, something like the following which is for incident notes. Note my pedantic checks for empty attributes are just there to make sure things still work if people change their minds about attributes being mandatory or not.
import System static def GetAttributeValue(Incident): Hours = 0.00 Minutes = 0.00 for Item in Incident.Notes: if Item._TimeSpentHours != null: Hours += Item._TimeSpentHours if Item._TimeSpentMinutes != null: Minutes += Item._TimeSpentMinutes return Hours + Minutes/60.00