This isn't based on any scientific info... or any studies or anything, it's just a personal preference. The longer the lab block is in the tank the more it appears to obsorb moisture, get darker, and ultimately loose its freshness. Take a fresh piece and break it in half. then take a piece that has been in the cage for 2 weeks and break it in half. There is definately a difference!
How long is too long? any longer then reasonably necessary.
Tell me, when you have to go into the cage to fill the water bottles every three or four days anyway, why would you want a hopper that holds more then a weeks worth of food?
Mike