I've been running my pellets for about 18 months now. I still have my original 500ml of EcoBAK and they have only slightly dissolved. I imagine it depends on the bioload. My friend Buganddoug has had to add to his over the same time, but his bioload is way more than mine.
I haven't opened the reactor since I added them. If they start to clump a little, I just increase the flow, then decrease it.