It can not be defined in terms of percentage. Percentage of what? %age of maximum take off fuel based upon aircraft structural MTOW or based upon Regulated TOW for the day, upon the fuel load at departure (usually less than max on short haul) etc. etc.
Minimum landing fuel is an absolute figure, based upon the requirements listed above by Gary.
On a day when your alternate is just 30 nautical miles away it might be 5%. On a day when it is 300 miles away it will be much more.