It is unlikely that Walmart will cash a check that is six months past its issue date. You can call their check cashing hotline 1-479-204-2125 and find out. It might depend on what type of check and who it is from. It is probable that you will have to contact the issuer and get a new check - but find out first.
Although there is no firm rule about this, most banks will not cash a check that is more than 6 months old. The only exception I know of is tax return checks - right on the check it says "valid for one year" and I've cashed them when they were 8 to 10 months old.
I don't know Walmart's policy on 6-month-old checks, but in my experience, stores are usually more stringent than banks about cashing checks.
For more information about check cashing, see this page: http://www.strangequestions.com/question/875/When-do-checks-expire-How-long-do-I-have-to-cash-a-check-after-is-it-issued.html
Posted 1083 day ago