My policy is I'll use e500s on 2 hour or less flights and I'll use CR1s or miles on flights > 3 hours. Flights from 2-3 hours I will take a look at the loads to gauge my chances of getting an upgrade and if the loads are light, I'll use e500s, heavy CR1 or miles. So far this year, I am 100% on my scheduled flights and about 75% on flights where I changed the schedule last minute (my own doing, so nothing to get upset about). And I still have plenty of e500s to burn thru, but none of them have expired so far. So I think my strategy works well for my flying habits.

I can do this b/c a lot of my flights are on UX w/o Explus, so I'm not able to upgrade all of them. I would not have enough e500s if I tried to upgrade every single flight I take. But UX solves that problem for me. I hate the ER145.