Depending on the warmth of your kitchen, the amount of milk and the amount of grains, it can be ready to 'drink' in anywhere from 12 to 24 hours.
It won't hurt you to drink it sooner (but there is more lactose in it) or later (it'll just be more sour, but the lactose content is gone or almost gone).
Here is the first page of the info sheet (so you have one to send with any grains you might pass on) that I send put with my grains; I'd posted it here, but it was way back..:
Here is an exact copy of the first page I send. I send some detail info sheets also regarding stuff like the dehydrating of grains for back up, etc:
KEFIR INFO SHEET
Here are your LIVE Kefir Grains. They have been shipped in a bit of whole, organic milk. The milk will be ‘Kefir’ by the time you get it. It might be on the ‘strong’ side, so I suggest gently straining off the Kefir milk, discarding it and placing your grains in a fresh jar of milk, following the below instructions.
In a clean, wide mouth glass jar (ie, a mason jar is wonderful), place these grains and 1-2 cups milk (whole, 2%, skim, pasteurized or not, homogenized or not – organic is preferable, though). Start with the smaller amount of milk, you can increase it over a few days time, as your grains grow.
Place a lid on the jar or a layer or two of paper towel or a piece of cloth over top (secured by a rubber band). Leave sitting on your countertop, out of direct sunlight for 24 hours. During the brew time, gently swirl the jar to make sure the grains are ‘bathed’ with the milk and this will help feed them and convert the milk to Kefir.
12 - 24 - 48 hours later, depending on milk to grain ratio and ambient temperature in your kitchen, you will have ‘real’ Kefir. It will be a bit tart and tangy. You will need to adjust the ‘brew’ time to get it to taste best for you. Less time will be less tart and more ‘yogurty’, longer will be sourer tasting.
Just prior to straining, the jar is gently shaken (swirled) or the contents are stirred to redistribute the curds back with the layers and pockets of clear-whey. This makes straining a little easier. You may not have curd/whey separation, if you brew for a shorter time period; no worries, it’ll just be more like yogurt.
Straining: Use a non metal strainer (I took a piece of the plastic mesh bag my garlic comes in, and cut it to fit over the lid of my mason jar, then secured it with a rubber band. This works well for keeping the grains from slipping through and it’s easy to clean. If you must use a metal strainer, try to avoid as much contact with the grains as possible. The acid content of the grains/Kefir can cause minute electrical currents that can harm the grains.
After straining, the grains are placed straight back into a pre washed fermenting vessel, without rinsing the grains. Fresh milk is added to the grains to prepare the next batch. As a buffer, a portion of fresh kefir from the previous batch, may be left [or added] in the fermenting vessel with the fresh milk plus kefir grains. This is how kefir was prepared traditionally in the Caucasus region. [I now use a nylon mesh strainer that was a kid's bath toy...hehe...it works great!]
The strained kefir is either consumed fresh, or poured into a sealed container and stored either in the refrigerator or stored at room temperature to ripen for a few days.
Every few days I usually place the grains in a little cup with some Kefir and give the ‘brew jar’ a thorough cleaning with mild dish soap and hot water (and rinse VERY WELL in fresh water). Do not add your grains back into a hot jar, it will kill them. The ‘Dom’ says this isn’t necessary, but it makes me feel better to give the brew jar a good wash occasionally.
Eventually you will notice the grains increasing in mass, and you can add more milk to the jar for brewing or remove some of the grains to give away or make a ‘back up’ copy.