Try making the rasam powder in this quantity, so it stays fresh. If you make a lot, freezing it would likely preserve freshness and potency, but I always make roughly this quantity and it lasts me about a month or so, depending on how frequently I make it.
I generally use roughly 1 tbsp of this rasam podi to make about 2 cups of rasam. It may seem like a lot, but my rasam is tangy with the tamarind and the tomatoes, so we need the heat and the flavor of the spices.
The podi can also be made with no oil at all, I just use a minimal amount to fry the spices.
Since the spices are basically fried and then powdered, they don't need simmering in the rasam for a long time, and once the tamarind base loses its rawness, about 5-8 minutes, your rasam is pretty much done! In fact, cooking for too long would change the taste of the rasam completely and it would get a very concentrated taste. I know, because I used to simmer my rasam for hours in my early days of learning to cook! And then wonder why my rasam tasted so strong and concentrated!
You can also make this podi for just one serving. Use about 1 tbsp of coriander seeds, and adjust all the other ingredients. The fresh podi tastes amazing if you fry the spices in a few drops of ghee instead of oil.
The general ratio of ingredients:
Coriander:Toor dal:Pepper:Cumin - 4:2:1:1.
With the red chilies, use as much or as little as you like, as the heat is determined by the variety. In general, the guntur red chiiles (the fat round ones) are hotter than the long red ones, but then, the current batch of long red chilies I have is super hot.
With this rasam podi, you're well on your way to passing the South Indian rasam test. Good luck!
Basic Rasam Recipe using this podi.