Kalindd's post is entirely correct and covers how trickle charge works. Yes, to both questions: there is a developer assembly for each color and it does occasionally need replacing depending on your usage. And yes, there is developer in the toner, the trickle part of trickle charge, to continuously top-off the developer assembly. The small amount of developer that gets sent to the waste container may or may not be old developer, there is no way for the little auger that dumps it to differentiate, but a constant fresh supply is still very helpful.
To answer the original question, the developer material and assemblies are both "technically" rated for 1.5 MILLION prints. Of course YMMV due to your environment and printing patterns.
Incidentally, I have two of this model, and I have found them to be shockingly long-lived. Both of my machines have over 1.5mil on the meter (about 2/3 color 1/3 black on both machines) and of the C, M and Y developers, NONE have been replaced yet. (Neither the material nor the assembly itself.) The K on one machine is also original, but the K on another machine needed replacing. (Just the material, assembly was fine.)
And when with them being that old, I still get beautiful clean smooth screens. It must be my operating environment I figure. San Francisco gots the good weather! Not too dry, not too humid, and never hot so no need for a moisture-sucking A/C to be running. (Xerographics do like a certain degree of humidity for the process to work best. Just not too much.)
FYI for nerds like me who care... A fresh supply of developer is about 420g in each color. The developer to toner ratio in the DEVELOPER mix is about 90% carrier, 10% toner. The ratio in the TONER is about the opposite, 90% toner, 10% carrier. Both are approximates.
