How many frames does a 'worn' camera have on its back? 10,000? More? Less?
When should you decide not to buy a used camera, frame-wise?
My girlfriend and I shoot something like 3000 frames a month. Should we stop taking 'pointless' photos that we know won't be great? Any good tips to reduce wear and tear on your camera?
10k shutter actuations are not that many. Most cameras today have an expected shutter lifetime of 50k or 100k. Professional models can have 300k or more.
And if the shutter breaks, it can be replaced (not cheap, but cheapter than a good camera).
So there is no reason to avoid taking picture. You bought that camera to take pictures, so that's what you should do with it!
The two biggest dangers to cameras are:
Avoid these, and the camera will likely last long enough for you to replace it volountarily because there is a newer model that does something you want.
A nice summary of current Nikon DSLR shutter ratings is at Shutter Actuations. The low end cameras are rated for 100,000 actuations while the high end is 400,000. I'd bet modern Canon's are in the same range, too. Nikon and Canon are pretty forthcoming with this information now; that wasn't always true.
Older cameras typically have shorter a shutter life -- low end in the 50k range; high end in the 250k range. But, it's often harder to find this info for old cameras.
The shutter is the part that wears out from actuations, but typical numbers of actuations are normally quite high (50 to 100k+ on a cheap shutter) and replacing a shutter is generally a small fraction of the cost of a new camera. The big thing when buying a used camera is you want to know when it will need a new shutter to know how it should impact the price you pay for it.
As for your usage, you should be fine shooting 3000 in a month. I do wedding photography and will shoot 3000 photos in a day. I do have a higher end camera that fails about 1/3 as often as yours, but at 3000 shots a month, you would still be using it for years before you would need to replace the shutter.
To give you an idea, even on my professional camera, the cost per time I press the shutter release is still primarily the cost of storing the image. The cost of each shutter actuation is measured in small fractions of a penny, so putting a number on it, 3000 photos only really costs you about $7 in shutter costs (rough estimate based on 100,000 shutter count failure and $250 replacement cost). That's less than you would spend buying and developing a single roll of film.
I tend to think of shutter actuations like mileage on a car. 100,000 is pretty much the wear-out point for a lot of folks, but statistically, there are mirrorbox assemblies that will break down before and after that count, of course.
One good source for information on this thing is the Camera Shutter Life Expectancy Database, where someone gathered data on the shutter counts of cameras that have died. This might give you an idea of how usage patterns vs. breakdowns tend to go on dSLRs.
I should also state my first broken-down dSLR, a Canon Rebel XT, bit the dust after about four years relatively hard usage, and it was not the mirrorbox or shutter that broke--it was the power board. The digital electronics are just as likely (or more) to be an issue than the mechanics of the camera when it comes to breakdowns.
