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One mistake I had when implementing Auto-renewable Subscription (a kind of In-app purchase) is not understanding how the sandbox environment works.

That resulted in wasted hours in debugging..

You should know this:

Time flies much faster in sandbox than actual. So a 1 week subscription will be only 3 minutes in sandbox. This actually helps because you can test your app much faster.

Actual duration	Sandbox duration
1 week	3 minutes
1 months	5 minutes
2 months	10 minutes
3 months	15 minutes
6 months	30 minutes
1 year	1 hour

Also, it will automatically renew maximum 6 times.

UPDATE: WWDC 2018 seems to change to maximum 5 renewals

Here’s a screenshot right from WWDC 2018: