Вы можете попробовать это
Models.py
class YourModel(models.Model):
deadline = models.DateTimeField()
def save(self, *args, **kwargs):
if self.deadline is None:
now = datetime.now()
num_days = 0
while num_days < 15:
now = now + timedelta(days=1)
if now.isoweekday() not in [6,7]:
num_days+=1
self.deadline = now
super().save(*args, **kwargs)