One advantage of having 30 day listings is that people watching your items don't have to renew them every 7 or 10 days when your listing is renewed. They still have to do them every 30 days, unless you have them on GTC (good till cancelled). You pay every month with GTC, unless you've got less than 200 listings.

If you have similar items you could think about using multi variant listings to save on listings.