How do I know when "adb backup" is finished?

I have a non-rooted Verizon Samsung Galaxy Nexus, and I wanted to try the "adb backup" feature. Possibly in preparation for rooting, but that's not really relevant to the question.
I want to back up everything.
I used this command on a Windows 7 command line:

adb backup -apk -shared -all -f c:\mybackup.ab

My phone prompted me for my password, which I entered, and clicked the button to begin the backup.

I let it run for several minutes, and the cmd window returned to C: prompt. On the phone, it was still flipping through filenames.

At some point, it stopped flipping through filenames but the back up selection buttons were still greyed out. C:\mybackup.ab existed, and was about 1GB in size, which is significantly smaller than I thought it should be, especially since I said to back up the apk files and the whole "shared" space (/mnt/sdcard).

What should I see (on the PC and on the phone) when the backup process completes? How long should I wait, if I estimate I'm using about 10GB of the 32GB of space?

Edit: ... and/or is that the wrong set of command line arguments?

Answer

The adb backup command won't work fully on an unrooted device as it will attempt to backup things like the /system partition, and without root access the phone will deny you this for safety reasons. I'm glad to hear running it with the -nosystem flag worked out for you :)

If you want to do a complete backup you need to be rooted. The /system partition, according to this page:

contains the entire operating system, other than the kernel and the ramdisk. This includes the Android user interface as well as all the system applications that come pre-installed on the device. Wiping this partition will remove Android from the device without rendering it unbootable, and you will still be able to put the phone into recovery or bootloader mode to install a new ROM.

This means that your backup does not include anything related to your operating system, only your apps and their settings. If you decide to root your device, you can create a very solid backup from CWM Recovery where all partitions are backed up to the sdcard on the device, including the system and boot partitions.