Simple demo code illustrating the advantages of enumerateSubstringsInRange over componentsSeparatedByString.

gistfile1.m

//
//  Created by Nikolai Ruhe on 2013-05-09.
//  Copyright (c) 2013 Savoy Software. All rights reserved.
//

#import <Foundation/Foundation.h>

static NSString *replaceLongWords(NSString *originalString, NSString *replaceWord, NSUInteger maxChar)
{
    NSMutableString *result = [NSMutableString stringWithCapacity:[originalString length]];
    __block NSUInteger location = 0;
    [originalString enumerateSubstringsInRange:(NSRange){0, [originalString length]}
                                       options:NSStringEnumerationByWords | NSStringEnumerationLocalized | NSStringEnumerationSubstringNotRequired
                                    usingBlock:^(NSString *substring, NSRange substringRange, NSRange enclosingRange, BOOL *stop) {
                                        if (substringRange.length > maxChar) {
                                            NSString *charactersBetweenLongWords = [originalString substringWithRange:(NSRange){ location, substringRange.location - location }];
                                            [result appendString:charactersBetweenLongWords];
                                            [result appendString:replaceWord];
                                            location = substringRange.location + substringRange.length;
                                        }
                                    }];
    [result appendString:[originalString substringFromIndex:location]];
    return result;
}

static NSString *replaceLongWords2(NSString *originalString, NSString *replaceWord, NSUInteger maxChar)
{
    NSMutableArray *array = [[originalString componentsSeparatedByString:@" "] mutableCopy];
    for (int i = 0; i < [array count]; i++) {
        NSString *str_ = [array objectAtIndex:i];
        if ([str_ length] > maxChar)
            [array replaceObjectAtIndex:i withObject:replaceWord];
    }
    return [array componentsJoinedByString:@" "];
}

static NSString *replaceLongWords3(NSString *originalString, NSString *replaceWord, NSUInteger maxChar)
{
    NSString *overSixCharsPattern = @"\\b[\\w]{7,}\\b";

    NSString *result = [originalString stringByReplacingOccurrencesOfString: overSixCharsPattern
                                                                 withString: replaceWord
                                                                    options: NSRegularExpressionSearch
                                                                      range: NSMakeRange(0, originalString.length)];
    return result;
}

int main(int argc, const char * argv[])
{

    @autoreleasepool {
        NSString *text = [NSString stringWithContentsOfURL:[NSURL URLWithString:@"http://www.catb.org/jargon/oldversions/jarg447.txt"]
                                                  encoding:NSISOLatin1StringEncoding
                                                     error:NULL];

        {
            CFAbsoluteTime start = CFAbsoluteTimeGetCurrent();
            NSString *result = replaceLongWords2(text, @"–", 6);
            CFAbsoluteTime end = CFAbsoluteTimeGetCurrent();

            NSLog(@"componentsSeparatedByString: time: %g", end - start);
            NSLog(@"result: \"%@\"", [result substringToIndex:[result length] > 1000 ? 1000 : [result length]]);
        }
        
        {
            CFAbsoluteTime start = CFAbsoluteTimeGetCurrent();
            NSString *result = replaceLongWords(text, @"–", 6);
            CFAbsoluteTime end = CFAbsoluteTimeGetCurrent();
            
            NSLog(@"enumerateSubstringsInRange: time: %g", end - start);
            NSLog(@"result: \"%@\"", [result substringToIndex:[result length] > 1000 ? 1000 : [result length]]);
        }
        
        {
            CFAbsoluteTime start = CFAbsoluteTimeGetCurrent();
            NSString *result = replaceLongWords3(text, @"–", 6);
            CFAbsoluteTime end = CFAbsoluteTimeGetCurrent();
            
            NSLog(@"stringByReplacingOccurrencesOfString: time: %g", end - start);
            NSLog(@"result: \"%@\"", [result substringToIndex:[result length] > 1000 ? 1000 : [result length]]);
        }
    }
    return 0;
}