"This reaction requires XYZ activation energy, therefore it cannot happen at room temperature." is a sentence I find often in organic chemistry textbooks. Activation energies are steadily given in textbooks too. Even though it is a nice piece of info, which gives a feeling of how (not) sluggish a reaction can be, it would be nice to know how much energy can such a system actually supply? Like, I don't know - 30 kJ/mol or 35 kJ/mol. How much energy do particles in a system at normal conditions (25°c, 1 atm) have? I am note sure if there even is an answer, taking into consideration Maxwell-Boltzmann distribution, and an endless array of reactions... but maybe there is?
Echoing Jon Custer's comment:
R = 8.314 J/K/mol
Multiply by 298 K, and you have a rough-and-ready rule-of-thumb number:
The "thermal energy" at room temperature is about 2.5 kJ / mol.