They expire after a year but you can acrue till then if you want.
To use them, you generally have to let your cashier know, "I'd like to use my points" or whatever, and they just take it off. I've found though you have to usually let them know you even have a card to scan, haha.
Check your point totals at the bottom of your receipt though, and you'll see a # there, and like 7000 points = $7 I think. Something along those lines.
Where I live, WM is at least 25 or 30 minutes away, and Target is 45 or more.
K-Mart is 5 and 20 minutes... So I shop there a bit out of necessity, but it's paying off in toys. Some of their stuff is reasonable, they have sales on figures that actually kick ass sometimes, and they actually stock stuff when other stores often aren't. I've seen all of Wave 3, 4, and 5, and 6 at K-Mart locally. They often only get 1 or 2 cases of any wave, but they get them in. And at $9.99 they're cheaper than TRU's BS.
I did an oil change when they had high mileage oil on sale and filters one week, and so that was points on my card. They had paper towels on sale, more points. They had sunscreen on sale before I went on vacation, and more points for me. And to me, employees at Target and WM are pretty much just as helpful as KM. I never have much luck, but fortunately I don't ask many questions either, haha.