How long should a child politely wait before interrupting adults?

Obviously interrupting can be rude but how much long does a kid have to wait for the conversation to end and give them your attention.

During my childhood and early tren years, my parents were always into "don't interrup when adults are speaking" but many times they would on and on and by the time it was over (after like 30 minutes or so), I wouldn't be interest anymore, felt ignored and I would simply forget the topic.

Should a child or anyone wait too much to finally get acknowledged? What if the conversation is taking too long?

How long should a child politely wait before interrupting adults?
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