How long is a SQS MessageID globally unique?

From FAQ - How long can I keep my messages in Amazon SQS message queues?

Longer message retention provides greater flexibility to allow for longer intervals between message production and consumption. You can configure the Amazon SQS message retention period to a value from 1 minute to 14 days. The default is 4 days. Once the message retention quota is reached, your messages are automatically deleted.

So message would only be to be associated to a specific message ID for a maximum of 14 days.

The message ID is a UUID, and as such, it is considered unique, although theoretically, they can repeat.

From Wikipidia: When generated according to the standard methods, UUIDs are, for practical purposes, unique. Their uniqueness does not depend on a central registration authority or coordination between the parties generating them, unlike most other numbering schemes. While the probability that a UUID will be duplicated is not zero, it is generally considered close enough to zero to be negligible.

What do you need it for?