Thanks everyone, you actually gave me some ideas to go off of in terms of additional research, and it looks to be an issue with Ink Purging, combined with other factors:
Ink Purge - When the printer is idle for 2 days or more, it cools down completely. When the printer exits the cool-down mode after being idle for 2 days, it purges ink as part of the warm-up cycle. The printer is ready to print in about 20 minutes. This also seems to be referred to as a "Full Warmup"
http://www.office.xerox.com/userdoc/P300X/users/06chapt7.html (different printer but seems like it explains the problem).
1) Learning Usage Patterns (thanks Eric) - I suspect the system was seeing usage once a week on different days, and is sometimes coming out of powersave - doing an Ink Purge. We have turned this feature off.
2) Power Outages - we occasionally have power outages - once every other month. This will cause an ink purge.
3) Since we only print ~1 time a week. The printer needs to do a ink purge when it prints too.
Looking at how much ink ends up in the tray after an ink purge, we are probably using a decent chunk of a stick a week. I cannot find numbers on the amount of ink, but it definitely looks like quite a bit judging by the pictures in the Repair Manual.
Looking through the 8550 manual , I see nothing about leaving the printer off for a long period of time.
And after doing some more searches I came up with this:
Note:You should leave the printer on
if you use it frequently (such as
during a normal work week with daily
use). The printer has an automatic
standby mode that keeps the printer
ready to use and reduces energy
consumption. If the printer will be
idle for two days or more (such as
over a weekend), turn it off.
So, I think to save some power, and avoid any accidental ink purges, I will keep the printer off when not in use.