I have been working on this question for a long time and I think most people have been fooled into believing an incorrect answer. My research indicates that it's actually 41.
From the best comic writer[0] how he found the answer to be a little more than 41:
The answer to this is very simple. It was a joke.
It had to be a number, an ordinary, smallish number,
and I chose that one. Binary representations, base
thirteen, Tibetan monks are all complete nonsense.
I sat at my desk, stared into the garden and thought
'42 will do'. I typed it out. End of story.
There is no amount of research that can take this away from me.
[0]: http://en.wikipedia.org/wiki/42_(Hitchhiker%27s_Guide_to_the...