S7-200 plc

I think 50 or 60 days. you can check from cpu manual

The super capacitor gives about 50 hours on the S7-221 & 222, about 100 hours on the S7-224 &226. Adding a battery gives a power down memory life of about 200 days. Or you can use this Library module from Siemens to write data to eeprom on power down which should hold the data for a very long time. Or if the data doesn't change during execution of the program you could store the data in the Data Block of the program or to an Eeprom cartridge.

Bryan

Bryan is right on the money.

The write to EEPROM logic is something that is fairly easy to implement. I use it on all programs that do not have battery cartridges.

The data block thing is also something you should ALWAYS do. When the application has finished commissioning and you want to store the current working values:
1. Put the PLC in stop mode.
2. Do PLC - Create Data Block from RAM

It will take the current working values in RAM and write them to the Data Block (write them to EEPROM) and then offer you a choice to upload this to your programming device. You should do this.

The way the data block works is if the PLC suffers a power loss that exceeds the capacitor or battery, the CPU will use the data block to write to RAM. So all those PLC's you put out there without a data block will come up with zeroes for setpoints if the capacitor or battery fails. If this could affect the process...