Using data from here: http://nssdc.gsfc.nasa.gov/planetary/factsheet/venusfact.html, increasing Venus' rotational speed to match Earth's would require about 1.5e29 J.

It's insolation is about 3e17 W, so assuming that somehow all this energy could be transferred to rotation, it would take about 16000 years - not absurdly long actually.

Using data from here, increasing Venus' rotational speed to match Earth's would require about $\:\mathrm{1.5\times 10^{29}\ J}$.

It's insolation is about $\:\mathrm{3\times 10^{17}\ W}$, so assuming that somehow all this energy could be transferred to rotation, it would take about 16000 years - not absurdly long actually.