Interactive Designer itself should not have any session timeout. However, if you are working from Pressero, there is I think a 2 hour session timeout after which your Interactive Designer session could not be added to the cart.
Forms based edoc within Pressero has the same 2 hour session timeout. Forms based edoc in an IFrame (integrated to your own ecommerce system, or testing from Creator) has a 20 minute timeout, I believe. That can be increased if it is causing a problem.