What if the condition_variable::wait_for delay parameter changes during the wait process?

I prefer using condition_variable::wait_for as a timer over chrono based timer since I can override the wait condition by specifying a predicate. So I wrote a test program to check what if the delay duration changes once the wait process has already started. The condition_variable::wait_for seems to ignore the change and instead comes out of the wait process entirely. Why is this? Can I ever change it in the middle of a wait process?

enum STATE {START,STOP,NONE};

STATE  state ;
condition_variable cv;
mutex mu;
chrono::milliseconds delay;

void cv_wait_thread()
{
    {
        unique_lock<mutex> lock(mu);
        cv.wait(lock, [](){ return state == START; });
    }

    chrono::time_point<chrono::high_resolution_clock> start_time = chrono::high_resolution_clock::now();

    {
        unique_lock<mutex> lock(mu);
        cv.wait_for(lock, delay);       
    }

    auto diff = chrono::duration_cast<chrono::milliseconds>(chrono::high_resolution_clock::now() - start_time);


    cout << "Conditional_wait:  "<< diff.count() << " milliseconds" << endl;

}


void main()
{
    thread thread1([](){ cv_wait_thread(); });

    state = NONE;   

    this_thread::sleep_for(chrono::seconds(1));

    delay = chrono::milliseconds(3000);

    state = START;
    cv.notify_all(); // ask thread to sleep for 3 sec

    this_thread::sleep_for(chrono::milliseconds(2000)); // let cv_wait thread start the wait process for at least 2 sec already

    delay = chrono::milliseconds(5000); // ask thread to correct and sleep for 5 sec instead

    cv.notify_all();

    thread1.join(); // thread prints 2000 milli
}