Doubt about my battery

Hello Apple Community,

I've some doubts about my MacBook Pro Retina (2012) battery.

Based on technical spec, my MBP can stay alive about around 7 hours on battery, however in real life, it can stay around 4 hours

Around 20% my mac goes into sleep mode and asks me to recharge it.

system_profiler SPSoftwareDataType && pmset -g batt

Software:

System Software Overview:

System Version: macOS 10.13.3 (17D102)

Kernel Version: Darwin 17.4.0

Boot Volume: OSX HD

Boot Mode: Normal

Computer Name: MacBook Pro de Florian

User Name: Florian **** (LEFBE)

Secure Virtual Memory: Enabled

System Integrity Protection: Enabled

Time since boot: 2:39

Now drawing from 'Battery Power'

-InternalBattery-0 (id=5177443) 30%; discharging; 1:13 remaining present: true

Battery information can be fount below:

system_profiler SPPowerDataType

Power:

Battery Information:

Model Information:

Serial Number:xxxxxxxxxxx

Manufacturer: SMP

Device Name: bq20z451

Pack Lot Code: 0

PCB Lot Code: 0

Firmware Version: 511

Hardware Revision: 3

Cell Revision: 1150

Charge Information:

Charge Remaining (mAh): 1801

Fully Charged: No

Charging: No

Full Charge Capacity (mAh): 6702

Health Information:

Cycle Count: 608

Condition: Normal

Battery Installed: Yes

Amperage (mA): -1380

Voltage (mV): 10616

My question is how can know when I need to change my battery ?

Thanks by advance for your help

LEFBE