Typically you want to keep your calcium levels down below 500ppm. I'm in the southwest where the fill water is very hard (my fill water has 280ppm CH minimum). So after two + seasons, my water is at 600ppm CH.

The main problem with high CH is the potential to scale or precipitate calcium onto your pool surfaces. The simple jingo for it is this - High pH + High TA + High Temperature is BAD. There is a parameter you can calculate called the Calcite Saturation Index (CSI) which basically describes the ability of your water to absorb or release calcium from solution. It's a logarithmic scale where -0.6 or lower means you water can be corrosive to a plaster surface (the water will absorb calcium) and where numbers larger than +0.6 means you water will potentially scale out calcium carbonate. All of your water parameters (pH, TA, CH, temperature, salt, borates, etc) all go into calculating the CSI value.

The numbers are just rough guidelines but they give a good measure of your waters balance with respect to calcium. There are people with pools that have CH values over 1000ppm but they are still able to maintain scale-free pools simply because they adjust their other water parameters to compensate and they keep their CSI values negative. Eventually, if the CH gets high enough, you can't reasonably lower the TA or pH any further and then you need to consider other methods such as water replacement or reverse osmosis treatment to remove calcium build up.

So there's no hard & fast rule that says a particular CH value is "too high" as it all depends on your other water parameters. You can find a VERY technical discussion of water chemistry and CSI calculations in this thread over at the TFP website