Run the following powershell script to determine the last logon date from the AD users.

Output will be written to c:\users.txt

# PSLastLogon.ps1
# PowerShell script to determine when each user in the domain last
# logged on.
#
# ----------------------------------------------------------------------
# Copyright (c) 2011 Richard L. Mueller
# Hilltop Lab web site - http://www.rlmueller.net
# Version 1.0 - March 16, 2011
#
# This program queries every Domain Controller in the domain to find the
# largest (latest) value of the lastLogon attribute for each user. The
# last logon dates for each user are converted into local time. The
# times are adjusted for daylight savings time, as presently configured.
#
# You have a royalty-free right to use, modify, reproduce, and
# distribute this script file in any way you find useful, provided that
# you agree that the copyright owner above has no warranty, obligations,
# or liability for such use.

Trap {"Error: $_"; Break;}
$file = "c:\users.txt"
$D = [System.DirectoryServices.ActiveDirectory.Domain]::GetCurrentDomain()
$Domain = [ADSI]"LDAP://$D"
$Searcher = New-Object System.DirectoryServices.DirectorySearcher
$Searcher.PageSize = 200
$Searcher.SearchScope = "subtree"

$Searcher.Filter = "(&(objectCategory=person)(objectClass=user))"
$Searcher.PropertiesToLoad.Add("distinguishedName") > $Null
$Searcher.PropertiesToLoad.Add("lastLogon") > $Null

# Create hash table of users and their last logon dates.
$arrUsers = @{}

# Enumerate all Domain Controllers.
ForEach ($DC In $D.DomainControllers)
{
    $Server = $DC.Name
    $Searcher.SearchRoot = "LDAP://$Server/" + $Domain.distinguishedName
    $Results = $Searcher.FindAll()
    ForEach ($Result In $Results)
    {
        $DN = $Result.Properties.Item("distinguishedName")
        $LL = $Result.Properties.Item("lastLogon")
        If ($LL.Count -eq 0)
        {
            $Last = [DateTime]0
        }
        Else
        {
            $Last = [DateTime]$LL.Item(0)
        }
        If ($Last -eq 0)
        {
            $LastLogon = $Last.AddYears(1600)
        }
        Else
        {
            $LastLogon = $Last.AddYears(1600).ToLocalTime()
        }
        If ($arrUsers.ContainsKey("$DN"))
        {
            If ($LastLogon -gt $arrUsers["$DN"])
            {
                $arrUsers["$DN"] = $LastLogon
            }
        }
        Else
        {
            $arrUsers.Add("$DN", $LastLogon)
        }
    }
}

# Output latest last logon date for each user.
$Users = $arrUsers.Keys
ForEach ($DN In $Users)
{
    $Date = $arrUsers["$DN"]
    "$DN;$Date" | Out-File $file -Append
}

Original script (without export to file) from: http://www.rlmueller.net/PowerShell/PSLastLogon.txt