To get a tourist visa, you need to get a reference code. Iran Travel Service gets you the code in less than 2weeks. Of course, if you are American or British that may take a bit longer. After getting the code, you can refer to your preferred consulate in person and get the visa in one single day. Otherwise, you can use the mail service. But the conventional posting may take about 2 weeks. As the visa is valid for entry 3 month from the date of issuance, it is wise to apply for your visa one month earlier to make sure that you will get it in time not at the eleventh hour.