When do the Macbook Pro fans turn on?
The short answer: We can't tell you exactly.

The algorithm that controls this is sure to be Apple-proprietary. I'm sure they've spent lots of work perfecting it, so they wouldn't share it for other hardware manufacturers to 'take'.

Basically, Apple's algorithm tries to strike a balance between not turning on the fan when unnecessary, like for a short spike in load, (so the computer is quieter and the battery lasts longer) and not letting things get too hot. Just how hot things are allowed to get probably depends on the components in your computer, the various temperatures of those components, how well the computer is ventilated at the moment, the ambient temperature, and maybe even things like load average.

Bottom line: If the computer's doing a lot of work, the fans go on to cool it off. There's no way to say exactly what temperature triggers the fans because it varies based on a lot of things.

Sorry I can't provide anything more concrete.