I have a model rocket I'm planning to launch. I heard from someone that you can calculate the altitude by timing the rocket from the time it takes off to the time it lands. Does anyone know how to do this?
You can decouple the horizontal and vertical motion of your rocket. In the vertical direction you have vertical thrust and gravity and horizontally you only have thrust (I ignore air resistance here). As you are interested in the altitude only, we only look at the vertical problem. All kinetic energy in the vertical direction is converted to potential energy so that your rocket reaches a height $h$.
Since the acceleration is constant, you have $v=at=gt$ and we find that the height equals
Since your rocket travels both up and down and since the path is symmetric around the apogee, you can calculate your altitude by
where $\Delta t$ is the time between launch and landing.
Did model rocketry for a while... from a practical standpoint this isn't feasible... unless you're talking about deploying something that drops at a rate you know at the time of the parachute deploying (like a weighted streamer. See the link below). Your chute deploys and you don't know the descent rate of it, so there's no way to find the vertical distance.
However, a more practical thing is to track the path of the rocket upwards, from a distance away from the base (say, 400 feet, maybe more for a more powerful rocket). Use a clinometer to track the angle the rocket flies up and record the maximum angle. From there, use trigonometry to find max height.