Well I recently swapped out my 280 for a ATI 6950.
AnandTech - Bench - GPU11
If you look at this chart, you'll notice that the GTX 285 is outclassed by a GTX 4601gb which can be had for around $100.
AnandTech - Bench - GPU11
Then you can see that the GTX 560ti for $150ish is much faster than the 285.
But to answer your question... the best time to upgrade is when YOU feel that the performance isn't good enough anymore. If you want to max out Battlefield 3 for example, then you'll need to upgrade.