Shelf-life of a Rogaine/Alopexie bottle?

I bought some Alopexie (EU Rogaine equivalent) at 5% with a prescription from my dermatologist.

As I am a woman, I was told to only use it once a day. I know a bottle is normally meant to last 1 month with a 2x a day use and the instructions say to discard the bottle after a month.

Well, I'm coming up to a month now and my bottle is of course still half full. I don't know whether I should throw it out or keep using it. Did anyone hear from their dermatologist on how long the product can keep?

Thanks for the advice :)

EDIT: it's the 5% liquid type (not foam).