Each license is valid for a single domain (not including sub-domains).
If you have PrestaShop 1.5 & 1.6 and use the "Multi-Store" is not necessary to pay more licenses. When you go shopping, you should place the domain of the main store.
The license and functionality of the module is a lifetime, ie after the first 6 months, you can still use the module without free updates and without support, although the ideal is that you always have our latest version and our support for possible problems or questions.
To purchase the upgrade and expansion of support for 6 months (180 days) will have to buy this expansion paying 50% of the cost of the module (more details).