What is the maximum key length of a tag?

Maximum key length: 128 Unicode characters
Maximum value length: 256 Unicode characters
Case sensitive
Maximum number of tags per resource: 50
Maximum active tag keys for Billing and Cost Management reports: 500
Reserved prefix—aws:
AWS-generated tag names and values are automatically assigned the aws: prefix, which you cannot assign. User-defined tag names have the prefix user: in the Cost Allocation Report.
Use each key only once for each resource. If you attempt to use the same key twice on the same resource, your request will be rejected.
You cannot tag a resource at the same time you create it. Tagging requires a separate action after the resource is created.
You cannot backdate the application of a tag. This means that tags only start appearing on your cost allocation report after you apply them, and do not appear on earlier reports.
Allowed characters are letters, whitespace, and numbers, plus the following special characters: + – = . _ : /
Note
If you need characters outside this allowed set, you can apply standard base-64 encoding to your tag.