Answer: It depends on how close the coast is from the epicenter
There are "local tsunamis", which are formed near the epicenter of the earthquake and it takes only a few minutes to reach the coast and there are tsunamis whose epicenter is distant (due to an earthquake far away from the place) and it can take up to 22 hours to reach the coastal areas.
For example, in the earthquake ocurred in Japan in 2011, there were areas that were farther from the place where the tsunami was generated, so the inhabitants had between 15 and 20 minutes to evacuate, however, in other places the wave took only 10 minutes on landfall and the inhabitants had only 3 minutes to evacuate.