SQS message retention and visibility

There are two important attributes (http://docs.aws.amazon.com/AWSSimpleQueueService/latest/APIReference/Query_QuerySetQueueAttributes.html)  set when working with AWS SQS.  The first is the MessageRetentionPeriod and the other is the VisibilityTimeout.  These are two very different and distinct attributes.  The MessageRetentionPeriod is the length of time (in seconds) the message will stay in the queue (unless it is deleted).  The value can be 60 (1 minute) to 1209600 (14 days).  The VisibilityTimeout is the length of time no other applications can NOT see the message while an application is processing the message.  This can be set to be 0 to 43200 (12 hours). The longer the time this is set the longer you expect one process to be working on this message.

SQS automatically deletes messages that have been in a queue for more than maximum message retention period. The default message retention period is 4 days. However, you can set the message retention period to a value from 60 seconds to 1209600 seconds (14 days) with SetQueueAttributes

More on why messages are not deleted once read and the visibility of a message while it is being processed by an application:
http://docs.aws.amazon.com/AWSSimpleQueueService/latest/SQSDeveloperGuide/AboutVT.html