This article deals with the validity of visit visas, Validity before and after stamping and validity after travel and the extensions allowed.
To know how to get a visit visa Please see http://qsaudi.com/getting-visit-visa-for-your-family-and-parents-online/
Validity Before stamping
Once a visit visa is issued you can get it stamped with in 6 months from date of issue,
Validity after stamping before travel
Visit visa is usually valid for travel for 3 months after stamping, Once Stamped the visitor must travel within 3 months.
Validity and extensions after travelling
And regarding the validity of visa and extension of stay in Saudi Arabia, it depends on the first validity that you get when the visa is stamped.
If you get a visit visa stamped with 30 days, then it can be extended 6 more times after arrival in Saudi Arabia, that is you will get a total of 7 months in the Kingdom
And if the visit visa is stamped 90 days at first time it can be renewed 2 more times, that is the visitor will get a total of 9 months (270 days).