New Corolla plugs last upto 40k kms without any problem. If you want to be more cautious or if you have an extreme driving style or if you are using sub-standard fuel or if you have extra money then you can opt to change at 20k. No problemo!
As far as warranty goes, have you ever visited dealers backyards? Have you ever noticed accident damaged, un-registered cars there. Ever wondered where they end up?
The reason why I was saying that this warranty thing is a scam is because this all depends upon your relationship with the particular dealer. No matter how religiously you follow their instructions or suggestions or recommendations, if you are ever in a situation where you need to get a warranty claim, it will always depend upon your personal relationship with the dealer. Or vice versa, if you are in good terms with him, he will never bother about maintenance schedule or spark plugs or genuine parts; he will give you your claim even if he has to replace your part with a new part from another "zero meter" car from his stock.