As per @TechnicallyWell said about 500 deep cycles so its more like 1000 partial cycles. So how long it lasts is a function of how much you use it. The lipstick type size which are popular because they are smaller capacity are going to tend to experience more deep cycles because their small capacity necessarily tends to empty them, so would be expected to last in the 2 years period. The bigger ones like you use less often and being larger tend to not experience a deep cycle so often, will tend to last longer. The bigger ones which are stored will also last longer if they are stored 60% charged, then fully charged before use, so help ensure the higher cost of the larger powerbanks produce the maximum useful life.
Also what happens slowly over time is your device's internal battery which the powerbank recharges, has slowly diminishing capacity so as the device (say mobile phone) ages (towards 2 years) it needs an external powerbank more often. That Powerbank will also be slowly dimishing its capacity over time so an older device with older powerbank will become less viable over time but an older device with a newer powerbank is a good combination.