In the trend (historical trend) screen that is displayed in the SMARTDAC+ GX/GP series web application screen, how far back can it go?

The web application maintains a buffer for holding historical trend data. The buffer can hold 3600 data points per channel.
Also, you can click the Get historical trend data icon and change the region of data that the buffer holds.
For details, see "Trends: Displaying Historical Data" in section 3.1.4, "Monitoring the GX/GP Data and Controlling the GX/GP from the Monitor Screen" in IM 04L51B01-01EN.