With J-Field you can currently adjust the coordinate of the base and then recalculate the coordinates of all those points collected from that base coordinate.
To do this go to Points and choose a point surveyed from that Base position. Click the edit button and then the button containing the Antenna Height. It will then list the statistics for that point, click the Base button and then enter the adjusted position and click ok. You will be prompted if you want to correct all related surveyed points, choose yes and all your points will be updated using their vectors from the OPUS coordinate you entered.
Any calculated points or imported points in the database will NOT move with the change in base coordinates. This is reasonable since there is no connection between the base and a calculated point, but it should be understood and considered when using this technique.
Also, when changing the base coordinate, the default system you will be presented for the base coordinate will be WGS84/ITRF2008. You can change this to show any coordinate system in the library and then edit the coordinates.
For the time period, I let the base station record the data the entire time I have it set up and am using it as a base.
Accuracy improves with time. The minimum time for OPUS Rapid-Static is 15 minutes.