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We have a form where mobile users submit their mobile number to receive an SMS with a PIN, which they have to use as confirmation in an online form. The PIN expires in a few minutes, so a user can request a new one. To avoid sending multiple SMS to a user, the system waits for some time before sending a new one to the same number. Each SMS sent costs money, so we're trying to optimise the amount of time between requests.

How long is reasonable to ask a mobile user to wait for an SMS? Is there any data that I could use to back this decision up?

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Firstly, I would review the system that sends the SMS and discover the average time it takes for those texts to be sent. More efficient systems send it immediately, others may take a few minutes, other may fail very often. So inspect the quality of the service.

Let's say it takes on average, 50 seconds. Give the system a few minutes grace period (I don't have an exact formula but let's take 3 minutes).

The message displayed on the form should be: "You will receive your PIN within 3 minutes." After 3 minutes if they haven't input the pin yet, display a "Send again" button so another pin is sent.

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