what is the limit of substr in oracle code example

Example 1: plsql substr

SUBSTR( string, start_position [, length ] )

Example 2: Subtr Oracle ?

/*Using SUBSTR in Oracle (Example from hackerrank.com): */

/*Simple select query...*/ 
SELECT DISTINCT city 
FROM station 

/*Using WHERE and SUBSTR to find (distinct) cities in station table that begin 
as well as end with a vowel.*/ 
WHERE SUBSTR(city,1,1) IN ('A','E','I','O','U') 
	AND substr(city,-1) IN ('a','e','i','o','u'); 

/*Parameters for SUBSTR (Substring) in order are as follows: 
String, Start, Length.*/