PopUpTimeChecker will check if something that happened last time exceeds a specific time duration.

ImageViewController.swift

import UIKit

class ImageViewController: UIViewController {
    
    override func viewWillAppear(_ animated: Bool) {
        super.viewWillAppear(animated)
        
        if let hiResFlag = hiResFlag {
            if hiResFlag == false {
                if PopUpTimeChecker.shouldShowPopUp() {
                    self.presentAlert()
                }
            }
        }
    }
    
    func presentAlert() {
        let alert = UIAlertController.init(title: nil, message: "Show Pop up", preferredStyle: .alert)
        let action = UIAlertAction.init(title: "Yeahh!", style: .default, handler: nil)
        
        alert.addAction(action)
        
        self.present(alert, animated: true, completion: nil)
    }
    
}

PopUpTimeChecker.swift

import UIKit

@objc class PopUpTimeChecker {
    
    //////////////////////////////////////////////////////////////////////
    //MARK:-
    //MARK:- Properties
    //MARK:-
    //////////////////////////////////////////////////////////////////////
    
    @objc static var popUpTimeInterval: UInt64 = 1296000 //15 days in seconds
    
    @objc static var hiResFlag: Bool? {
        get {
            return UserDefaults.standard.object(forKey: "HiResFlag") as? Bool
        }
        set {
            UserDefaults.standard.setValue(newValue, forKey: "HiResFlag")
        }
    }
    
    @objc static var isFirstTimePopUp: Bool {
        get {
            let value = UserDefaults.standard.object(forKey: "IsFirstTimePopUp")
            return value == nil ? true : value as! Bool
        }
        set {
            UserDefaults.standard.setValue(newValue, forKey: "IsFirstTimePopUp")
        }
    }
    
    @objc static var lastDateOfPopUp: Date? {
        get {
            return UserDefaults.standard.object(forKey: "LastDateOfPopUp") as? Date
        }
        set {
            UserDefaults.standard.setValue(newValue, forKey: "LastDateOfPopUp")
        }
    }
    
    
    //////////////////////////////////////////////////////////////////////
    //MARK:-
    //MARK:- Methods
    //MARK:-
    //////////////////////////////////////////////////////////////////////
    
    @objc static fileprivate func setLastPopUpDate() {
        //Setting current date to last shown pop up date
        lastDateOfPopUp = Date()
    }
    
    @objc static fileprivate func timeIntervalSinceLastPopUp() -> UInt64 {
        //Returning how much time (in seconds) has passed from last popup date until now
        return UInt64(Date().timeIntervalSince(lastDateOfPopUp!))
    }
    
    @objc @discardableResult static func shouldShowPopUp() -> Bool {
        
        //We proceed further only if we have the last date when pop up was displayed, else we create and set it as the current date
        guard let _ = lastDateOfPopUp else {
            self.setLastPopUpDate()
            return self.shouldShowPopUp()
        }
        
        let timeInterval = timeIntervalSinceLastPopUp()
        if timeInterval > popUpTimeInterval {
            self.setLastPopUpDate()
            return true //Show pop up
        } else {
            if isFirstTimePopUp {
                //If this is the first time, you just allow the pop up to show, don't allow otherwise
                isFirstTimePopUp = false
                return true
            } else {
                return false
            }
        }
    }
}