Never.

"The moment you doubt whether you can fly, you cease for ever to be able to do it."  -J.M. Barrie, Peter Pan

There is no shortage of doubters and naysayers.  They are an abundant and often outspoken lot, whose words have the ability to completely zap the energy from you.

"That's never going to work."

"That's not the way we do it here."

"You'll never finish before the deadline."

"Someone else is already doing it, so don't bother."

Negativity isn't always this easy to see and is sometimes wrapped and packaged as a nugget of wisdom.  Ever heard someone say that "the idea is worth nothing without execution"?  This expression is pseudo-motivational and attempts to elevate the importance of execution, but it does this at the expense of undervaluing the creation of a great idea.  It's impossible to execute on no idea, and I'm sure you can think of several well executed failed ideas.  Of course, even failed ideas have merit if you take the time to look.

The world has met its quota on negativity, so don't let self-doubt keep your ideas from execution.  There is fuel in failure and other's negativity.  These are abundant resources for powering new ideas.