XtraBackup vs Load on high transactional DB

I have a question, on an high transactional DB instance having ibdata size of about 330G and transaction per second ranging as
26.46 inserts/s, 15.83 updates/s, 0.00 deletes/s, 1030927.37 reads/s.
Pages read 10348128, created 1250698, written 10414639
6.03 reads/s, 1.06 creates/s, 2.31 writes/s
Buffer pool hit rate 1000 / 1000. If I had to take xtraBackup online. I know it will affect the load utilized in DB server but would it go worst on such high transactional DB server having 16G memory for which 12G is allocated to innodb_buffer_pool_size? Why I ask this is because I never had a way to stop mysql instance alone. Every time whenever I need a bounce I had to reboot the entire server.

Also any idea how long would it take to complete the backup?