Good question! I think the answer is when it is only a couple of shades darker than your skin and you have blended the edges very well. The idea of bronzer is to look like the sun has just touched the high places of your face- cheekbones, nose, edge of forehead, chin. The point is not to spread it all over so you look uniformly brown or worse, orange! Think of it more as a "reverse highlighter" and be sure to blend the edges so you don't have a muddy looking patch.
A trick I use when I contour my face with bronzer on my cheekbones is to apply the bronzer on my cheekbones but shade up instead of going down...so you'd lightly use a fluffy brush and instead of blending down use it upwards. A bronzer is buildable, so apply it lightly first and see how you look in natural lighting. Sometimes all you need is a light dusting to really liven up your face! Also, you can just ask someone! I ask my husband constantly if my look is 'too much' or 'cakey' and he always gives me his opinion even if I don't like it.
I completely agree with what prettyinpa said about how your bronzer should be applied naturally as if the sun has just touched the high places of your face (cheekbones, nose, forehead and chin) and not all over your face. I only wanted to add that using a fan brush makes it easy to apply a light touch of bronzer without applying too much. With a fan brush, you can apply, stand back and look and if you need more then you can easily keep building a little more color at a time. If you don't have a fan brush, here's an example of a good one to work with your bronzer: http://www.sephora.com/browse/product.jhtml?id=P14
I hope this helps you.