Prepaid Phones

I have a Mobily prepaid phone and it is causing some frustration. I got it one or two months ago and everything was okay. I then bought a 10 riyals card to top it up and received the message that it would run out in three days! A colleague at work said that a 30 riyals card lasts for a month. However, when I bought one today it said that it would expire on Jan 15th (Jan 3rd today).

Would it make a difference to buy a 50 riyals card? Do Zain or STC top up cards last longer?

1- Charging 10 Riyals will extend the validity 3 days.

2- Charging 30 Riyals will extend the validity 10 days.

3- Charging 50 Riyals will extend the validity 60 days.

4- Charging 100 Riyals will extend the validity 120 days.

5- Charging 300 Riyals will extend the validity 1 year.