Unfortunately, Li-Ion cells have a rather finite life. Deeper cycles reduce life more quickly than shallow cycles. Holding at a very low or very high SOC (state of charge) reduces life more quickly than holding at a 50% SOC. Higher temperatures also reduce life. In addition to being cycle-limited, they are also age-limited. Even if you don't use them at all, Li-Ion cells slowly deteriorate.
Cycle life is determined under lab conditions, not real-world conditions. A cell rated at 400 cycles will drop to 80% capacity in the lab when cycled continuously, but will invariably last many fewer cycles in the real world.
The life you achieve also depends on reliability. You've got a bunch of cells in the battery pack, each one with a different time to failure. When the first one starts to go, the pack is toast. The failure rate is defined statistically, so it could occur almost any time.
Rob