It depends what question you are asking. If the question is "should we fund projects even if we don't know if or when they will have demonstrable results?" then I would argue that, yes, some portion of research should go towards that.
How long is "long enough" to deem that a line of experimentation "didn't work out?" There is no period long enough. Sometimes a negative result is quite useful down the road. Some research just comes together when the right things are learned and tried. So any notion of a "dead end" is really just a tentative assessment, frozen in time.
Without harsh assessments, a massive project will eventually dedicate some of its resources into self-perpetuation, that is justifying its existence rather than producing results.
It's a risk we need to take (or mitigate) if we want to have a steady flow of new breakthroughs. Science, just as programming or any other creative discipline, is best done when you have more money than you need and nobody is looking at your hands.
How long is "long enough" to deem that a line of experimentation "didn't work out?" There is no period long enough. Sometimes a negative result is quite useful down the road. Some research just comes together when the right things are learned and tried. So any notion of a "dead end" is really just a tentative assessment, frozen in time.