How can I set a lifespan for a projectile?

public class ProjectileFlightTimeUntilDecay : MonoBehaviour
{
    public float PlayerProjectile;
    public float timeUntilDecay = 5.0f;

    void Start()
    {
        timeUntilDecay = timeUntilDecay * Time.deltaTime;
    }

    // Update is called once per frame
    void Update()
    {
        if(timeUntilDecay <= 0)
        {
            // Destroy the projectile
        }
    }
}

I tried this, and doesn’t work.

>Solution :

timeUntilDecay is never decreased from its initial value, so the check if (timeUntilDecay <= 0) will never be true.

Typically this is done by decrementing it in Update(), e.g.

timeUntilDecay -= Time.deltaTime;