How long approx should an iPad that hasn't been used in two years take to charge?

So I just pulled out my beaten up iPad that I stopped using two years ago because I screwed it up so much half the screen was unresponsive to touch. However it was still able to turn on and worked fine (at least half of it did).

I've plugged it into the charger now where it has been for 30 minutes with the same black screen with the depleted battery symbol on it.

I realise it will likely take a long time to bring back to life after having no juice what so ever for two years BUT if anyone has had similar experience, how long does it approx take?

Cheers

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