How much sun is too little?

Depends on the plant of course. But that is not a useful answer. If you see any of these symptoms in your house plant it probably needs more light.

Plant slowly drops older leaves
Older leaves are pale or yellow or a lighter green than they should be
Older leaves may be larger than normal, newer leaves smaller than normal

New growth is slow
New growth is spindly or weak

Plant bends toward the light source

No flowers
Bud drop
Flowers are pale or faded in color
Flowers are smaller than they should be

If you see any of these symptoms try putting your plant in a brighter window or adding light with a table lamp that has a fluorescent bulb.