A non-production instance is an add-on instance that is separate from your production instance(s). Its not a trial instances so its doesnt expire. All users in your production instance(s) are also licensed for all your non-production instances, unless you want to restrict access using a security group.
A question I get a lot, is "how do I know I have been granted a free non-production instance?"
After you have been granted a free non-production instance, when you log into CRM Online, you will see a yellow message bar with the message "Congratulations, You have qualified for a free non-production instance" and a "Configure" button.
You can always purchase non-production instances, currently at approximately a quarter of the price of production instances.
- Introducing Sandbox Instances in CRM Online - link