View source for The Time Used to Generate Fossil Fuels by the Sun

*Area of the earth that faces the sun is 49M square miles = pi * (3952 mi)^2
*1 square mile is  2.6M square meters
*Insolation from the sun is 1000W/square meter
*Efficiency of photosynthesis is 0.1%
*Energy stored in plants per day if earth surface was 100% filled with plants - 1000 J/second/sq m * 86400 seconds/day * 49^6 sq mi * 2.6^6 sq m/ sq mi = 1.1^22 J/day
*Reserves - 1 Teraton of oil equivalent is left globally - [http://www.iaee.org/en/publications/proceedingsabstractdoc.aspx?id=1092].
*Diesel is 50MJ/kg
*1 teraton = 1 tera 1000 kg = 1^15 kg 
*Teraton crude oil has 1^15 kg * 50^6 J/kg = 50^21 J = 5^22 J
*'''Thus, it takes 5 days of solar photosynthesis hitting the earth at a low conversion scenario of 0.1% to equal all remaining fossil fuel reserves of earth.