How do I know how much to put in my face cream or body lotion? How long does it keep? I'd like to make a lotion about spf 30. Thanks!

You can get powdered titanium dioxide at organic creations (google it) ... but I don't know how to manipulate the concentrations - I do think Maxx did some of the math on it, and it should be here in the forums somewhere if you do a search on titanium dioxide.

More than 10% of titanium dioxide (measured by volume) will make you look pasty.
I usually mix about 10%, that is borderline pasty.

Also, from my experience titanium dioxide will not mix well with glycerin based lotions. With petrolatum based lotions, as well as oil based lotions that's much better. (of course heavy concentrations of neither glycerin nor petrolatum are ideal for the skin but suppose you have a quick exposure to a hot summer sun and then you wash everything on your skin away, for instance when you do some sports outside: then some petrolatum lotion could work too, if you cannot afford to mix the powder with more expensive lotions).

You can use a hand mixer or a manual whisk to mix the powder with the lotion.

I have also tried titanium dioxide in pure squalane and it did not mix at all so you need a cream that allows for better suspension.