It would be possible for me to not design anything new for at least a year and I'd have many possibilities for design ideas for Zazzle products just using the designs I have already.
This is sort of what I have been doing lately. I am rediscovering some of my older designs that I had forgotten about but really liked! Using them in ideas for pattern mash ups, something I love, is quite fun! The odd time I have to go back into my PSD file and tweak the colours as it may not work quite right with the other designs I wanted to combine it with, but mainly, there is a huge array of patterns and designs available to me right in my image files of past designs on Zazzle :-)
Screen shot here of my most recent created products, some pattern mash ups and some just using one designed pattern on a new product.
Designing is so much more fun than listing so often, I'll use one of my designs on a few products, then move on. Revisiting is perfect! New ideas abound :-)