It took more than 15 years for Albert Einstein to be rewarded with a Nobel Prize in physics. A combination of factors contributed to the delay. At the time many of his theories were scientifically unproven, and other factors like his fame may have prevented him from winning the Nobel Prize sooner.
After Einstein's famous scientific papers were published in 1905, the theories described in those papers, which related to subjects such as the photoelectric effect and the theory of relativity, had to be peer reviewed and tested to see if they held any merit. This is part of the reason why Einstein didn't win the Nobel earlier in his academic and scientific career.
In the interim, while Einstein's published theories were subject to peer review, his international fame grew. The Nobel selection committee seems to have taken issue with this, and when his work was largely confirmed in 1919, the committee passed him over and gave the 1920 award to another scientist. Einstein felt confident that he would eventually win a Nobel Prize: part of his divorce settlement agreement with his first wife, Mileva Maric, included a provision that required Einstein to turn any Nobel Prize earnings over to Maric. This happened when he won the 1921 Nobel Prize.