nine months/months' jail

[Tan Elaine]

57—year—old Kum Chin Tiong, was sentenced to nine months/months' jail in January after he pleaded guilty to paying S$100 for sex with the girl.

Which of the above nouns in bold should I use?

Thanks.

 

[5jj]

It should be months', but not many people worry about it these days.

 

[emsr2d2]

... nine months' jail
OR
... nine months in jail.