In many cases when a reader puts a story aside because it 'got boring,' the boredom arose because the writer grew enchanted with his powers of description and lost sight of his priority, which is to keep the ball rolling.
I like to get ten pages a day, which amounts to 2,000 words. That’s 180,000 words over a three-month span, a goodish length for a book — something in which the reader can get happily lost, if the tale is done well and stays fresh.
If you're just starting out as a writer, you could do worse than strip your television's electric plug-wire, wrap a spike around it, and then stick it back into the wall. See what blows, and how far. Just an idea.