public class YourCoolMonobehaviour : Monobehaviour
{
    [SerializeField] private float cooldownTime = 3.0f;
    private CoolDownInformation cooldownMana;

    private void Start()
    {
        cooldownMana = new CoolDownInformation(cooldownMana);
    }

    private void Update()
    {
        // The "IsElapsed"-function has to run every frame
        if (Input.GetButtonDown("Fire2") && cooldownMana.IsElapsed(Time.deltaTime))
        {
            //Do your Mana Stuff
        }

        // If this is not possible, use "IncreaseByDeltaTime"-function to increase the elapsed time in the structure and the "IsElapsed(bool resetAfterElapsed = true)" for checking
        cooldownMana.IncreaseByDeltaTime(Time.deltaTime);
        if(sometimestrue)
        {
            if (Input.GetButtonDown("Fire2") && cooldownMana.IsElapsed())
            {
                //Do your Mana Stuff
            }
        }
    }
}

[System.Serializable]
public struct CoolDownInformation
{
    public float Cooldown;
    public float elapsedTime;

    private bool cooldownOnStart;

    public CoolDownInformation(float cd, bool cooldownOnStart = false)
    {
        this.Cooldown = cd;
        this.elapsedTime = cooldownOnStart ? 0.0f : (cd + 1.0f);
        this.cooldownOnStart = cooldownOnStart;
    }

    public bool IsElapsed(float deltaTime, bool resetAfterElapsed = true)
    {
        if (!this.cooldownOnStart)
        {
            this.elapsedTime += deltaTime;
            bool state = this.elapsedTime >= this.Cooldown;

            if (resetAfterElapsed)
            {
                this.elapsedTime = 0.0f;
                return state;
            }
            else
            {
                return state;
            }
        }
        else
        {
            this.cooldownOnStart = false;
            return true;
        }
    }

    public bool IsElapsed(bool resetAfterElapsed = true)
    {
        if (!this.cooldownOnStart)
        {
            this.elapsedTime += deltaTime;
            bool state = this.elapsedTime >= this.Cooldown;

            if (resetAfterElapsed)
            {
                this.elapsedTime = 0.0f;
                return state;
            }
            else
            {
                return state;
            }
        }
        else
        {
            this.cooldownOnStart = false;
            return true;
        }
    }


    public void IncreaseByDeltaTime(float deltaTime)
    {
        this.elapsedTime += deltaTime;
    }
}