NSString* source = @"11001200"; //test length = 6
NSString* hh1 = [source substringWithRange:NSMakeRange(0, 2)];
NSString* mm1 = [source substringWithRange:NSMakeRange(2, 2)];

NSString* hh2 = [source substringWithRange:NSMakeRange(4, 2)];
NSString* mm2 = [source substringWithRange:NSMakeRange(6, 2)];

NSString* results =[NSString stringWithFormat:@"%@:%@-%@:%@",hh1,mm1,hh2,mm2];