(1a) I lost my job. However, I have enough money for two months' living expenses.
(1b) I lost my job. However, I have enough money for two months' of living expenses.
(2a) Please give me two months' notice before you move out.
(2b) Please give me two months of notice before you move out.
(3a) Jack owes two months' rent.
(3b) Jack owes two months of rent.
(4a) I spent two months' salary on my new car.
(4b) I spent two months of salary on my new car.
Some of my friends think it's correct to say "two months' of living expenses". For the rest of my sentences, you say "two months' notice/rent/salary. I don't understand why. Please explain that.