What are the limitations to a free account?
I believe 10 devices, but what other limits?
Still 180 days of data?
You can easily find these limits, including the data TTL applied to your account, at the bottom of your Sandbox Overview page.
Typical sandbox TTL is 30 days, not 180 days. And generally speaking, you are correct about 10 devices being the limit. However, some users with older accounts or other agreements may have higher per-sandbox limits.
As for other limitations, there are two features that are only available to organization users: