Now that searches are organized by relevancy, renewing your listings for exposure isn't really a good option to get exposure.. Only if the searcher changed to "recently listed" will the results show that way. I am so glad because constantly renewing to get at the top of the search was costly and often not worth it.
I just renew when the listing runs out or I sell it.
I try not to renew very often, after all it's an additional $0.20 per renew. But if I haven't had any views in a day or two I usually renew one of my more popular products to get people coming back and at least looking.