J.S.
Lactose-free milk lasts a lot longer than regular milk because it doesn't have the lactose sugars in it to speed up the spoiling process. Smell it. If it smells spoiled like regular milk does, or "off" then you toss it.
If the soy milk gets "too thick" or it gets a weird oily sheen which I like to refer to as "pre-silky chunks" then toss it.