There is a way to use Internet Download Manager for more than 30 days. The only way you can do it is through purchasing. The software is shareware, therefore it needs purchasing before you can use it for 1 year. You can do this on the official website.
A good alternative is Free Download Manager. It works the same as IDM, has good integration and besides that, it's free meaning that you can use it for as long as you want.
Download page: http://www.freedownloadmanager.org/