My understanding is that there are a limited number of charge cycles, but it is related how much the battery recharges. I think that for most LiOn batteries, there are about 500-700 full charge cycles (before the battery starts to lessen in its capacity), but that is based on a full charge. However, if that battery is recharged from 50% to 100%, that is only 1/2 a charge cycle, so if you recharge at 50% each time, you basically get 1000-1400 charge cycles. Waiting to recharge the phone until it is 90% dead (100% discharge isn't good for LiOn batteries), will use up almost a full charge cycle, while recharging the phone 2x from 50% to 100% will use up 1 full charge cycle.
So I don't think there is much benefit to trying to run the battery longer. I guess the only way to really extend the life of the battery is to minimize the actual use of the battery, which kind of defeats the purpose of having the device.
There is new technology that looks to extend the numbers of charging cycles, and reduce the time to re-charge, hopefully by the time this battery wears out, and the phone is replaced, some of that new technology will be in the new phones 2 years from now!