Lactose milk doesn’t last as long as regular once it’s opened though.
Not sure what you were trying to say above, but regular milk contains lactose .. lactose-free milk does not.
My experience over the course of the past three years is that lactose-free milk stays good for approx. two months.
As an example, I bought a half gallon carton two or three weeks ago and the exipery date on the top says 'Good Til 08 May 2020'.
The previous carton I purchased (and opened) in mid-January had an exp. date of 26 Mar, and I just disposed of the small remainder last week.
I don't use much milk (obviously), and the lactose-free variety costs about half again as much as regular, but for my needs it's far and way the more economical buy.