Hi all, thanks for the response,
I am not part of the RSM club so my tank works a little bit different, regarding the filter sponge, would you believe that if you changed it more frequently it wouldn't hold as much bacteria therefore your tank wouldn't possibly go through a "mini cycle" or "crash" because of you changing it.
Your other media is meant to hold the bacteria, your filter sponge is simply a filter sponge that filters the rubbish out of your water before passing through the rest of your filtration, if it is allowed to collect a large amount of good bacteria, I would say it has been allowed to collect a large amount of bad nutrients also.
This is just a thought as it sounds like the longer it is left unchanged the more biological impact it takes on your tank when you do eventually change it.
Thank you for your responses as it has educated me, let the discussion roll on I say. Maybe someone else can learn something new along the way....