The average is about 35% at 1000 hours, with some projectors being as low as 15% to 20%, and some being as high as 50% to 75% loss. Most newer projectors will still have more than 50% of their brightness at 1000 hours though. In my experience, most projectors will be between 25% to 45% loss at 1000 hours if you run them in ECO, not counting lamps that dim too fast (like a defective JVC lamp).
Lamp ratings also mean diddly squat. My Viewsonic Pro8200 lamp was rated at 5000 hours, but at 3500 hours it is DIMMER than my Benq w7000 lamp which has about 2500 hours on it (technically the end of life of the lamp). The w7000 lamp is rated half the Viewsonic Lamp (2500 vs. 5000 both in eco), yet the Benq lamp dimmed at a slightly slower rate.
Too dim just depends how bright you start out and how lucky you get, for my Benq w7000, I started out at about 35fL to 40fL which was uncomfortable to watch in the dark but used the manual IRIS mode to tame it down until the lamp wore in. After 2500 hours on the Benq w7000, I am at about 20 to 25 fL now which is about 60% of the original lumens. So the w7000 lamp was rated at 2500 hours, yet at 2500 hours I still have 60% of the lamp's brightness left.
On my JVC RS-45 lamp, the first bulb dimmed to about 50% or maybe slightly worse around 1000 hours, then it exploded at 1500 hours. On my second RS-45 lamp, it dimmed 40% in only 350 hours (defective), and I should probably replace it soon so it does not also explode. On the Epson 5010 I used (though not at my own house), it had dimmed to about 50% of the original lamp at about 1500-1800 hours if I remember correctly.