Gregor Mendel published his paper on inheritance in 1866. It was ignored. His results were rediscovered by Hugo de Vries and Carl Correns in 1900, thirty four years later.
Charles Darwin formed his idea of natural selection in 1838. In 1842, he sent a letter to Charles Lyell outlining his idea. In 1858, Alfred Russel Wallace sent Darwin a paper describing natural selection. So there was a gap of sixteen or twenty years between the first discovery and the second.
Johannes Gutenberg started printing about 1450. Other people, such as Laurens Janszoon Coster were working on printing, but they were trying and failing.
Christopher Columbus convinced Ferdinand and Isabella of Spain to pay for his trip to America. He was looking for a way to get to Asia. He grossly underestimated the distance. Partly it was because he thought the Earth was 25% smaller than it actually is, and partly because he thought Asia extended farther around the world than it actually does. His estimate was probably in the range four thousand to nine thousand kilometers: the actual distance was twenty thousand. The experts at the time had a much better idea of the actual distance. Ferdinand and Isabella ignored them because they were desperate for money and Columbus looked like a possibility.
If Gutenberg or Columbus had failed, how long would it have been before someone else had discovered printing or America? In modern times, with more people searching with more resources, the delay between the first discovery and what would have been the second discovery has undoubtedly been getting smaller. In the fifteenth century, it would have been greater. A reasonable guess would be about a century. A delay of that size for either of those discoveries would have changed history beyond imagination.