How long do you take lactulose for, or does your motion then become regular or do you need to take?

Lactulose is a type of sugar. It is broken down in the large intestine into mild acids that draw water into the colon, which helps soften the stools.
It may take up to 48 hours before you have a bowel movement after taking lactulose. It is important that you drink lots of water while taking lactulose.
Lactulose is used to treat chronic constipation and can be taken safely for long periods of time.

Given that your husband is always constipated I would think your doctor is intending him to take it long term. Check with your doctor at your next visit.

See https://www.drugs.com/mtm/lactulose.html