In response to the OP, the panel appears to generate 100mA so, in bright direct sunlight and ignoring charging efficiency, it will power the RPi for one seventh of the time you left it in the sun. More like two thirds that because of the charging efficiency, and much much less if it isn't in direct sunlight. Light is logarithmic; if you think it's a little dull, there's probably only about 5% sunlight reaching you. Hazy sunlight, window glass or indirect sunlight will significantly increase charging time, cloud cover will increase it by ten times or more, and artificial light would be worthless.
Which does not mean that it is unreasonable, but you should probably plan on using a larger panel, like of the order of square feet.