I just watched "Man of Steel", and I'm wondering if my logic is correct.
Let's assume Superman is 80 kg. The energy required to take off from the rest to reach the speed of sound in air (if I neglect the drag) is:
$E_k = 0.5mv^2$ = $0.5\cdot80\cdot340^2$ = $4\times10^6 \ J$.
Also add the potential energy at height $h$, $E_p=mgh = 784h \approx 0.2\times10^6 \ J$ (Let's assume at $h = 300 \ m$)
Total energy is roughly $4.8\times10^6 \ J.$
Superman gains his energy from the sun. Assume solar flux at Earth's surface is $1340 \ W/m^2$ (max), and Superman's surface area is roughly $2 \ m^2$ (calculated using Du Bois formula). Then the maximum energy that can be absorbed by Superman on Earth is $2\cdot1340 = 2680 \ J$ per second. (Solar flux is much less on the surface, but here I used this value anyway.)
Then to take off, he needs to wait:
$(4.8\times10^6)/2680 \approx 1791 \ s \approx 29.9 \min.$
It doesn't seem correct. Please correct me if I am wrong. (This is close to the perfect situation, which neglects many factors that could make the charging time longer. This also assumes his tank is empty. Thanks for pointing it out. If you are interested, please feel free to write down a more realistic estimation.)