On the Grayl website they allege that you get the filter are designed to last 350 cycles. While I havent counted how many cycles I have used mine for, I am sure its pushing that. I primarily used mine in Idaho out of an assortment of rivers and creeks. As other have stated, it does get harden to push and filter the longer you use the filter, but I find that I don't actually have to push that much when I lean my body weight into it. My wife used her filter along side me, and while she didn't use hers as much, she had no problem cycling the water even as we approached the end of the filter's life.
I will note that I never actively tested the water that I was filtering to see how effective the filter was - I just figured as long as I stay under that 350 number and can still push it down, the filter is very likely doing what it needs to do.
Long story short, the geopress is great. Sure, its a bit heavier than other options, but you can also mix drinks in it, dont have to wait for the filtering to happen, and the construction is much more durable than some options out there.