It is very hard to add too much good compost, I don't think you will have that problem. But, a healthy soil does have a good amount of mineral content. If what you are amending is mostly subsoil, the ratio of compost/soil can be very high.
When organic matter decomposes, it makes humic acid, which is commonly used in labs to break apart minerals. Subsoil is almost all mineral, but soil tests often show a very small part of those minerals are available to plants. This makes most people want to add powdered minerals to the soil, to bring up the number to where it should be, not realizing that all the minerals are there, all they need is to apply organic matter, especially with some that is still actively decomposing. This will drastically increase the amount of available minerals in the existing soil. The humus rich material will also aid in drainage and water retaining capacity, as well as providing the plants with the ideal root zone.
So it's important that there is still some mineral soil mixed in, at least for most plants. And it helps to have decomposing plant matter on the soil at all times, like quickly decomposing mulches (grass clippings come to mind), to keep the available mineral content up where it needs to be for best plant health.
Also, as decomposing plant matter produces acid, compost is often acidic, so I'd test the soil after the compost has been added, to see if it is at a good level for the growth of the plants you intend to cultivate in that area.