Now, exactly HOW MANY attempts did Edison make to build a working light-bulb?

He probably could not answer that question either.

"It is said he tried over 6,000 different carbonized plant fibers, looking for a carbon filament for his light bulb."
Source and further information:
http://www.graceproducts.com/edison/life.html

"He invented the light bulb after innumerable failed attempts and when asked why he pursued his quest even after 10, 0001 attempts, not wanting to admit failure to which he simply replied, "I have not failed. I've just found 10,000 ways that won't work.""
Source and further information:
http://www.buzzle.com/articles/thomas-alva-edison-inventions.html

"Like Edison responded when asked if he would give up after ‘failing’ 700 times to create the filament for the light bulb… He responded that he did not fail, but instead discovered 700 ways that didn’t work."
Source and further information:
http://www.persistenceunlimited.com/2007/07/how-many-failures-are-you-willing-to-endure/

"He had tried a thousand different elements and all had failed. A colleague asked him if he felt his time had been wasted, since he had discovered nothing. "Hardly," Edison is said to have retorted briskly. "I have discovered a thousand things that don't work.""
Source and further information:
http://www.codinghorror.com/blog/2005/05/success-through-failure.html

Further information:
- "Is it true that it took Thomas Edison 999 tries to light a bulb?"
http://www.answerbag.com/a_view/6549381 (my own answer)

According to Nikolai Tesla (who worked for Edison for a while)....too many.

"If Edison had a needle to find in a haystack, he would proceed at once with the diligence of the bee to examine straw after straw until he found the object of his search. [...] I was a sorry witness of such doings, knowing that a little theory and calculation would have saved him ninety per cent of his labor."

http://refspace.com/quotes/Nikola_Tesla


http://en.wikipedia.org/wiki/Nikola_Tesla

I'm not sure because I feel asleep and lost track.