When I got my Visit Visa to Saudi, the Visa was valid for 90 days. I had to travel to Saudi within that time period and also my visa was valid only for a month. You can actually extend your visa, while you are here (considering that the person who sponsored you here apply for it). I came to Saudi on 9th June and the Visa was valid for a month till 9th July. My mother paid Visa extension fee which was 100 riyal and it got extended for another month.
Method: In order to pay your visa extension fee, you have to do through Sadad payment system. Go to an ATM and select following steps: Sadad system–> MOI—>Family visit visa fee . After you have paid the fee, your sponsor need to take the receipt to her/his mudeer ( For example, My mother is hired by a hospital so she took the receipt of the ATM to the Human resource department and showed them). Afterwards, they would take your passport and receipt and give you back maybe the same day. The passport will have a scribble date and a stamp showing that your visa has been extended. The procedure is very simple and you can pay the visa extension fee at the nearest ATM.
If your visa is for a month, you can get visa extension on Visit Visa numerous times. In order to reconfirm your visa validity and extension, you can ask Saudi embassy or travel agent.
Not to Forget: Pay the fee on time and also show the paid fee receipt to mudeer on time or else you would have to pay the fine which is 500 riyal (sucks!). Although I had paid the fee on ATM on time but i didn’t have my passport for few days so my mother was told by the mudeer to come back when you will have the passport with you. We came back after 3 days (just because we were out of town), the mudeer said that as now it is too late, we have to pay the late fee charge via the same system. We tried showing that the fee has been paid via ATM on time but he still said that we had to pay late fee charge (ugh!). The reason the mudeer gave was that the passport goes to immigration office (in the same city one resides in) and so as we are late, we have to pay the late fee charge via ATM and give him the receipt. After 2 days, my passport came back, scribbled with another extended date and on the brighter note, the visa was extended for 2 months and 10 days..
Hope it helps!