My mother in law has a 10 year visa to visit the USA. Question is can she stay with us for 4 months or is there a limit?
The maximum stay as others have indicated is six months. That stay can be extended for an additional six months under narrow extenuating circumstances, i. e., caring for a sick family member.
Note however those who do multiple entry almost six month stays over the course of the 10 year visa now run a heavy risk of denial when time for renewal......unless they can also establish a valid reason or reasons for those stays. Immigration is aware that many come here and work illegally...then return home for a 6 month period....then return. That has now become a red flag.
Respectfully,
Playacaribe2