IRCC deem full time work as 30 hours per week. So 30 x 52 =1560. So if you are working part time you need to total the number of hours to a maximum of 30 hours in each week. Divide the total by 30 and round down to the nearest week. So if you work 20 hours a week you could complete the 1560 hours in 78weeks, 25 hours per week in 62.4weeks.
jes_ON has given this answer in more detail several times.