My backups take up the majority of my SDcard (I have barely any music/movies on there), so I have plenty. It's easy enough to get into Astro (or mount via PC) and delete the old ones to save space. I generally backup everytime I add a theme/kernel or a new ROM. It takes a few extra minutes, but if you end up in a bootloop, you can always restore one step back, instead of one MONTH back.