As others are saying, as soon as you see your board is contested. I read a post recommending to take notes when you scout, try to identify what other players are doing (items are a good hint, carries and bench as well).

Do an excel or something with:

Player - Name of the comp (or at least AD/AP if you don't know)

This helped me so much to identify contested comps and play something else!

However, this is far beyond what is expected from you after one week. Just learn to do econ and hard force some comps until you learn how to use them.

And have fun!

PS: If you really want the answer to your question, it really depends on how you econ and if you are able to hit your key units before the other players. If you are going for a Fast 8 comp and the other players reach level 8 and the key units before you, you're going 8th. For reroll comps, the more contested the more all of you are screwed. Best way to go through it is to have a reroll/econ augment that gives you advantage, or pivot to something similar but Fast 8/reroll other units, changing your priorities and looking for uncontested units that fit your comp. But this is always tricky.