Schedule ship date based on quanity

Summary:

We have Requirement to default the schedule ship date when create Drop ship SO automatically with each line

if the quanity <=10 then lead time should be 3 weeks (schedule ship date)

if quanitity is 11 - 36 then lead time should be 2weeks

if quantity is > 36 then lead time should be variable.

Could you Please let us know how we can achieve this?