What Wrong with this Picture: Really Dirty Furnace Filter

You describe your filter as "a large 6 month filter." It sounds like you have a four inch thick High Efficiency Particulate Air filter, more commonly called a HEPA filter. A HEPA filter is an excellent filter, much better than the one inch thick monthly filters that you are familiar with. I have a HEPA filter on my furnace and it is designed to be replaced yearly. My opinion is that these filters are the best ones available for the money. Anything better would require electric power, and who needs another electrical appliance? If your filter says to replace it every six months, then do so. If it says nothing at all, go the manufacturer's website and look it up. Most of the ones that I see are yearly replacement designs. Thanks for bringing it up, I must change mine today.

Your condition does not appear to be normal. Maybe the ducts are full of dirt. Have them checked for dirt. Maybe dirt is being introduced into the system from an exterior source.