After you've soaked the grains 24 hours in milk and drained them off several times (and thrown away the kefir they made) - the grains should be plump and look like 'while plastic cauliflower') If that's what you see, then using 1T of grains for 1 cup of milk, your next batch should be fine to drink.
Actually, the very first time you did the 24 hour soak, it was not 'dangerous' to drink the fluid, it just likely didn't make "technically perfect creamy kefir" while the grains were still rehydrating. So folks recommend you throw it away for a few days until the grains are 'plumped up' and fully rehydrated.
Also, you can keep what's leftover in the fridge for a very long time.