Generally speaking, the visa is valid for 90 days from the date of issue at the embassy / consulate, certainly not the date the original application was approved by the MFA.
Once the embassy has received your visa authorisation, it will hold onto it for 1 to 2 months.
I have occasionally seen visas which are valid for only one month from the date of issue, though this is the exception rather than the rule.
Also beware that the embassy / consulate may take it upon themselves to issue you a visa for less than the 30 days you presumably applied for. They may give you as little as 10 days, though this too is unusual. However, so long as it is a tourist visa, you can extend it easily in Tehran for $20 (2010 price).
All the best, enjoy Iran, it's an awesome country.
Daniel
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