Good DIY mask/scrub? and how long to keep them?

[#1]

So I discovered a homemade mask I really like (chopped oats, honey, yogurt, a pinch of cinnamon/vanilla extract), and I usually just keep them in a 1/3 cup rameskin with saran wrap on top and use freshly cleansed finger every time (refregerated of course). It doesn't feel different, but the liquid is starting to separate after 2 weeks. How long can you keep a mask before it loses effectiveness?

I assume scrubs last longer, I mean, if you look at ingredients list for Fresh Sugar scrub, you can get most of them at health food store. I just use olive oil, brown sugar, and lemon zest (great lip scrub, too). But how long do you keep them?

Also, if you've got a favorite DIY mask/scrub, feel free to share. =)

[#2]

In all honesty, baking soda and water do the trick for me!  However, I avoid this scrub when I'm broken out because the scrub granules are too big/harsh on my skin, and tear open the pimples therefore letting bacteria in and making them worse.  But when skin is clear, this is an EXCELLENT scrub and is so cheap and easy!