The software is available for download which means that after you pay, it is downloaded to your computer. There is no period for how long the license is available for. You buy the application and a serial number/activation key is provided (is used to register it), then you can use the application for as long as you want.
However, the best way to obtain an official statement is through the developers of the application: http://www.surething.com/support/contact-support/
