Let me start by saying that I'm neither a fan nor a basher of Glocks in general. That said, I recently picked one up earlier this year, a Glock 19 gen 4, and found the gun and the sights serviceable. Due to the way the gun fits my hand, I found the gun to shoot slightly left (about an inch at 10 yards, which I chose to ignore. So I had no plans at all to change those sights, in spite of the fact they were plastic. Now I don't abuse my guns by throwing them, dropping them, jamming the front sight against things to cycle the slide, etc. Ever. I treated the gun the way I treat all of my guns.
About two months after buying it, it inexplicably began shooting more than three inches to the left. After some examination I found that the front sight was actually leaning slightly to the left. Further inspection proved that the plastic sight itself was actually slightly bent. I ordered a set of metal sights and had them installed; problem solved.
Is my experience common? I don't believe it is, even though there are a few reviews around the interwebs that describe nearly identical experiences. But these instances seem to be rare. In fact, out of everyone I know that owns a glock, I'm the only one that seems to have had this problem.
So I'd say that the odds are that the stock plastic sights on your gun will probably be fine for however long you own it. Just keep in mind that plastic is just simply not as durable as steel. So if they break/bend/whatever, no big deal, just get a new set.