I decided to try kefir instead of yogurt because it has more different probiotics and I didn't want to keep buying starter. Kefir is also more convenient because it cultures at room temperature, using lumps of probiotics and yeast that are called grains. I bought a kefir kit, but I found the strainer that came in the kit has too small holes, it is too hard to get the finished kefir through the strainer.
As with yogurt, I have been making my kefir with fresh raw milk from a source that I trust, without heating the milk first. With my raw milk from Jersey cows, the kefir sets up as firm as yogurt, and once the culture is going well a quart takes about 8 hours in this warm summer weather (my kitchen is in the high 70s or even 80). Raw goat milk makes a much thinner kefir with whey forming on top, instead of on the bottom as with cows milk, and takes longer. I use it in a breakfast smoothie so it doesn't matter how thick it is, but it is harder to tell when it is done.
My kefir grains seem to do fine spending most of the week in the refrigerator in some milk, though the instructions recommend keeping them at room temperature making more kefir all the time. After I left them in the refrigerator for almost three weeks when I was away, I restarted them by rinsing them and putting them in about a cup of fresh low-temperature-pasturized milk on the counter, changing the milk every 24 hours for a couple of days before going back to my usual quart of raw milk.
If you are thinking of getting into making kefir, make sure that you have a source of milk that is organic and not ultra-pasturized. And make sure you want to use kefir regularly; it isn't something you can make now and then when inspiration hits, the grains need to be used most weeks.