Hi. When working offline, all your data will be kept in your device. In terms of storage demands, data captured from ArcGIS QuickCapture is pretty light. The only factor that would increase your storage needs is photos. If you take many photos, storage needs will grow. You can control the size of your photos using the JSON editor in QuickCapture designer. The size of the photo is by default set to small, which is very conservative from a storage perspective. You can change the size to medium or large.
If you want to make a backup copy of your data, you can connect your device to a laptop using a cable and find all the data stored by the application under the QuickCapture app space. Having 20GB of space in your device looks like should be sufficient for three days of work. If you do not capture photos, I doubt you will need more than a few Mb. If you capture photos, with 20Gb you can capture many hundreds of them for sure, even if using a 'large' size setting.