您可以使用Handler.postDelayed（）将新线程调度为在x毫秒后运行;在该调度线程中，您将使用DownloadManager.query访问Cursor对象，该对象显示下载状态。如果下载状态指示下载未成功完成，您可以使用DownloadManager.remove（）取消它。
我测试了这个方法：

private fun tryDownloadFileButCancelIfTimeout(url: String, millisecondsToTimeoutAfter : Long) {
    val downloadManager = this.getSystemService(Context.DOWNLOAD_SERVICE) as DownloadManager
    val downloadUri = Uri.parse(url)
    val request = DownloadManager.Request(downloadUri).apply {
        try {
            setAllowedNetworkTypes(DownloadManager.Request.NETWORK_WIFI or DownloadManager.Request.NETWORK_MOBILE)
                .setAllowedOverRoaming(true)
                .setTitle(url.substring(url.lastIndexOf("/") + 1))
                .setNotificationVisibility(DownloadManager.Request.VISIBILITY_VISIBLE)
                .setDestinationInExternalPublicDir(
                    Environment.DIRECTORY_DOWNLOADS,
                    url.substring(url.lastIndexOf("/") + 1)
                )
        } catch (e: Exception) {
            e.printStackTrace()
        }
    }
    val downloadId = downloadManager.enqueue(request)
    // Schedule a new thread that will cancel the download after millisecondsToTimeoutAfter milliseconds
    Handler(Looper.getMainLooper()).postDelayed({
        val downloadQuery = DownloadManager.Query().setFilterById(downloadId)
        val cursor = downloadManager.query(downloadQuery)
        val downloadStatusColumn = cursor.getColumnIndex(DownloadManager.COLUMN_STATUS)
        cursor.moveToPosition(0)
        val downloadStatus = cursor.getInt(downloadStatusColumn)
        if (downloadStatus != DownloadManager.STATUS_SUCCESSFUL) {
            downloadManager.remove(downloadId)
        }

    }, millisecondsToTimeoutAfter)
}

发布我用来测试的代码（可以导入到Android Studio，如果你喜欢，一个简单的应用程序，一个下载按钮，开始下载，并在五秒钟内取消）：
https://github.com/hishamhijjawi/DownloadCancelDemoStackOverflow