Do you need to get back to your brewing Potion at exactly the right time or could you also finish it e.g. the next day?

You have an hour and 20 minutes to do part 2 after the gap of 80 or 100 or whatever minutes between the two halves. If you don't start the second part of the potion within that hour and 20 mins, the potion screws up. So no, you can't leave it for the next day.

I can't even find the potion I have brewing at the moment, is that normal if the time's not up?