How many days data does the SG6200NXL store on it without using a USB drive?

The SG6200NXL without a USB drive can store 1140 entries to be divided between the connected devices.

If you have a single device, this will last 19 hours. Two devices will last 9.5 hours. etc..

Adding a USB drive will increase this. A 1 GB USB drive can store close to a years worth of data.