If you dont mind using a commercial tool, take a look at CryptoLicensing It can limit licenses to N number of days.

You can also consider usage of 3rd party scheme like CryptoLicensing - it comes with built-in cryptographic LicenseProvider derived class for use with licx files.

Crypto Obfuscator supports obfuscating WPF assemblies. Try it out.

A good (and cheaper) alternative is Crypto Obfuscator

Crypto Obfuscator is a cost-effective and powerful alternative.