How long should it take to enable FileVault 2 on a fresh Lion installation?
You might find that the time taken relates to the size of the volume to be converted (not limited to the amount used). 

Personal estimates of duration may be difficult. 

Expect the most reliable estimates to be offered by System Preferences. If the system was only recently started, don't expect an accurate estimate of time; and don't be surprised if the estimate rises occasionally. 

For more detail than is given by the progress bar in System Preferences: in Terminal you can run the following command, which will display a tree view of the CoreStorage world:

diskutil coreStorage list

Within the tree you'll see sizes; amounts converted. 

If a volume is heavily used during conversion, it might slightly extend the time taken to complete but for your 10 GB used I would not expect there to be any difference. Use the volume as you normally would until conversion completes.