That's reasonable; I agree. For example, I'll wait just one more day.—Fair enough, you've been very patient. [Colloquial; early 1900s]
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.
Examples from the Web for fair enough
This was apparently a fair-enough answer and Fred bowed and walked away.The Rover Boys on Treasure Isle|Edward Stratemeyer (AKA Arthur M. Winfield)