Thanks George, I have changed the Log Backup Frequency to one hour.
I ran again DBCC SQLPERF(LogSpace) and it shows around 14 GB but I was looking Backup Log on my backup drive it also shows.
see the result:
Log Size (MB) Log Space Used (%) Status
14198.8 2.486978 0
I have just checked - select name,log_reuse_wait_desc from sys.databases and log_reuse_wait_desc shows Log_backup and also checking to VLfs, total files around 490 and 33 files with status = 2.
I have changed T-log backup frequency to every one hour, now backup log size is every hour is:
8 AM 7013 KB
9 AM 24897 KB
10 AM 45385 KB
11 AM 43220 KB
12 PM 419318 KB