function waitForUserInput([parameter(mandatory=$true)][int]$TimeoutSeconds, [string]$Prompt){
    $counter = 0
    while($counter++ -lt $TimeoutSeconds){
        if(-not [console]::KeyAvailable){
            Start-Sleep -Seconds 1
        } else {
            $input = Read-Host -Prompt "$Prompt"
            break
        }
    }
    if($input){
        Write-Output "$input"
    }
}

$ReportFileName = 'c:\whatever\some.file'
$UserInputTimeoutSeconds = 3

# ask user for custom report prefix
echo "Press any key in $UserInputTimeoutSeconds seconds to specify custom report name..."
$customPrefix = waitForUserInput -TimeoutSeconds $UserInputTimeoutSeconds -Prompt "Report prefix"
$customPrefix = $customPrefix -replace '[^\w]',''
if($customPrefix){
    $reportDir = Split-Path -Parent $ReportFileName
    $reportSuffix = Split-Path -Leaf $ReportFileName
    $ReportFileName = "$reportDir\${customPrefix}_$reportSuffix"
}

# ...