以下はアプリを起動した直後と以降は１時間以上アプリを離れて再度アプリに戻ると表示するやり方です。
AdmobAppOpenAd.swift
import UIKit
import GoogleMobileAds
@main
class AdmobAppOpenAd: NSObject, UIApplicationDelegate, GADFullScreenContentDelegate {
    var appOpenAd: GADAppOpenAd?
    let adRequest = GADRequest()
    let adUnitID = "ca-app-pub-3940256099942544/5662855259"//Test
    func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey: Any]?) -> Bool {
//       adomob初期化
        GADMobileAds.sharedInstance().start(completionHandler:{_ in
//        起動時に表示
            self.showAdIfNeeded()
        })
        return true
    }
    func sceneWillEnterForeground(_ scene: UIScene) {
        let interval = getElapsedTimeInSeconds()
        DispatchQueue.main.asyncAfter(deadline: .now() + 0.008) {
//この遅延が無いとなぜが開いてすぐ閉じてしまう
//      閉じていた時間が１時間以上、４時間未満のとき　（４時間で広告の使用可能期限切れ）
            if interval > 3600 , interval < 13500 {
                self.showAdIfNeeded()
            }
        }
    }
    func sceneDidEnterBackground(_ scene: UIScene) {
//        バックグラウンドに入る時にプリロード
        preloadAppOpenAd()
//        バックグラウンドに入った時間を保存
        saveCurrentTime()
    }
    func presentAppOpenAd(from rootViewController: UIViewController) {
        if self.appOpenAd != nil  {
            self.appOpenAd?.fullScreenContentDelegate = self
            self.appOpenAd?.present(fromRootViewController: rootViewController)
        } else {
            guard let windowScene = UIApplication.shared.connectedScenes.first as? UIWindowScene else {
                return
            }
            GADAppOpenAd.load(withAdUnitID: adUnitID, request: adRequest, orientation: windowScene.interfaceOrientation) { [weak self] (ad, error) in
                if let error = error {
                    print("Failed to load App Open ad: \(error)")
                    return
                }
                self?.appOpenAd = ad
                self?.appOpenAd?.fullScreenContentDelegate = self
                self?.appOpenAd?.present(fromRootViewController: rootViewController)
                print("AppOpen ad show")
            }
        }
    }
    func showAdIfNeeded() {
        guard let windowScene = UIApplication.shared.connectedScenes.first as? UIWindowScene,
              let keyWindow = windowScene.windows.first,
              let rootViewController = keyWindow.rootViewController else {
            print("ルートビューコントローラーが見つかりません")
            return
        }
        presentAppOpenAd(from: rootViewController)
    }
    func preloadAppOpenAd() {
        guard let windowScene = UIApplication.shared.connectedScenes.first as? UIWindowScene else {
            print("Failed to load App Open windowScene")
            return
        }
        GADAppOpenAd.load(withAdUnitID: adUnitID, request: adRequest, orientation: windowScene.interfaceOrientation) { [weak self] (ad, error) in
            if let error = error {
                print("Failed to load App Open ad: \(error)")
                return
            }
            self?.appOpenAd = ad
            print("App Open ad preloaded")
        }
    }
    var savedTime: Date?
    // 現在時間を保存するメソッド
    func saveCurrentTime() {
        let currentTime = Date()
        self.savedTime = currentTime
    }
    
    // 保存時間から経過した時間を秒数で返すメソッド
    func getElapsedTimeInSeconds() -> TimeInterval {
        guard let savedTime = self.savedTime else {
            return 0
        }
        
        let currentTime = Date()
        let elapsedTime = currentTime.timeIntervalSince(savedTime)
        return max(elapsedTime, 0)
    }
    // MARK: - GADFullScreenContentDelegate
    func adDidDismissFullScreenContent(_ ad: GADFullScreenPresentingAd) {
//       ここにプリロードを入れても初回起動時の広告を閉じたときは無効になる
    }
}