Just to add something more from the other people.
All the "time" information is always written (when known) in the single operators' pages here on PrepaidGSM.net under the word "
Validty". In the same cell are usually listed the different recharge vouchers too and their validity, credit, bonuses, etc...
There's not 1 single rule, there are hundreds of different rules depending on the Country, the operator, the tariff...
You just need the patience to read carefully the
National Operators pages. Then if you still have doubts you can ask here.