Where does Windows 8 store the estimated data usage over Wifi?

I remember when you first asked this question, but I finally got around to figuring it out. Hope it's still of use to you or someone else!

You can access the this data by calling the GetLocalUsage method of ConnectionProfile objects, which are WLAN/WAN connections (i.e. SSIDs). GetLocalUsage takes two DateTime parameters and returns a DataUsage object containing the amount of data sent and received during the interval specified. You can get a list of ConnectionProfile objects by calling the GetConnectionProfiles method of NetworkInformation.

I wrote the following function that retrieves the data and returns an object. Pass to it one or more SSIDs and optionally start and stop DateTime's:

function Get-EstimatedDataUsage()
{
    Param(
        [Parameter(Position=0, Mandatory=$true, ValueFromPipeline=$true, ValueFromPipelineByPropertyName=$true)]
        [ValidateNotNullOrEmpty()]
        [String]$ProfileName,

        [Parameter(Position=1, Mandatory=$false)]
        [ValidateNotNullOrEmpty()]
        [DateTime]$From,

        [Parameter(Position=2, Mandatory=$false)]
        [ValidateNotNullOrEmpty()]
        [DateTime]$To
    )

    Process
    {
        foreach($profile in $ProfileName)
        {
            try
            {
                [void][Windows.Networking.Connectivity.NetworkInformation,Windows,ContentType=WindowsRuntime]
                $ConnectionProfiles = [Windows.Networking.Connectivity.NetworkInformation]::GetConnectionProfiles() | Where-Object ProfileName -EQ $profile
            }
            catch
            {
                Write-Error 'Unable to create instance of Windows.Networking.Connectivity.NetworkInformation.'
                continue
            }

            foreach($ConnectionProfile in $ConnectionProfiles)
            {
                $ProfileName = $ConnectionProfile.ProfileName

                if($From -eq $null)
                {
                    try
                    {
                        $ResetTime = Get-ItemProperty -Path HKCU:\Software\Microsoft\Windows\CurrentVersion\Network\DataUsage\Wlan\$ProfileName -Name ResetTime -ErrorAction Stop | Select-Object -ExpandProperty ResetTime
                        $From_determined = [datetime]::FromFileTime($ResetTime)
                    }
                    catch
                    {
                        $From_determined = [datetime]::FromFileTime(0)
                    }
                }
                else
                {
                    $From_determined = $From
                }

                if($To -eq $null)
                {
                    $To_determined = Get-Date
                }
                else
                {
                    $To_determined = $To
                }

                $usage = $ConnectionProfile.GetLocalUsage($From_determined, $To_determined)

                $op = '' | select Name,Received,Sent,From,To

                $op.Name = $ProfileName
                $op.Received = $usage.BytesReceived
                $op.Sent = $usage.BytesSent
                $op.From = $From_determined
                $op.To = $To_determined

                $op

            }
        }
    }
}

This MSDN article is the best I could find from Microsoft: http://msdn.microsoft.com/en-us/library/windows/apps/windows.networking.connectivity.datausage.aspx. It tells you how you can call it from a program, but not where the data proper is stored. I'm not going to copy and paste it all because I don't know which language you prefer to program in.