Are you, perhaps, suggesting that the Eraser Scheduler display a status bar for in-progress tasks giving the percent complete and estimated time remaining? Nice idea although I do not know whether it is feasible and whether it is the most critical use of scarce Eraser resources.
The short answer to your specific question is, "A long time, possibly hours." The long answer is, "It depends upon the device, how it is connected and, especially, what Eraser options were selected."
First, does your portable device connect via SATA-I, SATA-II, SATA-III, USB 2.0, USB 3.0 or Firewire? And, if it supports SATA-III (or USB 3.0), was it plugged into a SATA-III (or USB 3.0) port? Modern systemboards usually have SATA-III and both USB 2.0 and USB 3.0 ports but the USB 3.0 ports are usually located in back, not the front. If your device is SATA-I (or USB 2.0) OR was connected to a SATA-I (or USB 2.0) port, ALL operations will take longer.
Finally, what Eraser options did you select? For normal users, over-writing with a single-pass of random numbers is more than adequate. However, if you were using an ultra-paranoid, government mandated 35-pass method, resign yourself to it taking a VERY LONG TIME! And, my last thoughts are that if you just wanted to wipe the device before using it, a FULL Format (NOT Quick Format) would have been more than adequate to get rid of anything on the device.