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The license is lifetime. You can keep it forever in your Steam account,

Terms of Usage

1. No piracy, no reverse engineering, not copying, modifying the product materials

2. Not selling the product again, sharing with someone

3. Not using the product for unlawful things

4. The product is sold as it is. The developer does not guarantee to add new features

5. Bug fixing period will be limited. In the future (For example when Windows 15 is released) the product may not work because of technical reasons.

6. The product is designed for generic Windows 7+ Operating System users. If the product owner has a specifically configured system, the product may not work. The product does not guarantee to work on all systems.