The amount of time a Cookie set by someone clicking on your affiliate link has to show a conversion before you are no longer credited with a sale even if that user eventually ends up making a purchase. The standard length of a Cookie is typically between 30-90 days. Anything below 30 is considered low/short while anything above 90 is considered to be healthily above average.
Once you've protected your prospecting pool, maximize your affiliate program by working with the best and leaving the rest. As the old 80/20 adage implies, most of your revenue will come from a very small percentage of your affiliates. Because it can be time-consuming to manage a larger affiliate network, consider selecting only a few companies initially, and interview them before signing them on. Affiliates are an extension of your sales force and represent your online brand, so choose partners carefully.