How much sun is too much?

That depends on the plant of course, but here are symptoms your plant will show if you are giving it too much sun:

Edges of leaves may turn white, beige or brown
Edges of leaves may be red
Tips of leaves may brown
New leaves smaller in size
New leaves are brown
Leaves may turn white, or gray
New leaves may be too pale in color
Entire plant wilts

And sometimes a plant will remain green but refuse to grow.

If you see any of these symptoms, and especially if you see more than one symptom, try moving your plant to a less sunny location.

House Plants Get Sunburn