Note that the graph is not linear, its a log-log graph!
Photocells, particularly the common CdS cells that you're likely to find, are not sensitive to all light. In particular they tend to be sensitive to light between 700nm (red) and 500nm (green) light.
Most datasheets use lux to indicate the resistance at certain light levels. But what is lux ? Its not a method we tend to use to describe brightness so its tough to gauge. Here is a table adapted from a Wikipedia article on the topic!
|0.002 lux||Moonless clear night sky|
|0.2 lux||Design minimum for emergency lighting (AS2293).|
|0.27 - 1 lux||Full moon on a clear night|
|3.4 lux||Dark limit of civil twilight under a clear sky|
|50 lux||Family living room|
|100 lux||Very dark overcast day|
|300 - 500 lux||Sunrise or sunset on a clear day. Well-lit office area.|
|1,000 lux||Overcast day; typical TV studio lighting|
|10,000 - 25,000 lux||Full daylight (not direct sun)|
|32,000 - 130,000 lux||Direct sunlight|