What would be a reasonable amount of time needed for some arthropod to evolve into megafauna, given the right evolutionary pressures?

Roughly how long would arthropods have to evolve before they could reach sizes analogous to current day megafauna (like today's mammals)?

To solve the problem with large arthropods, assume these megafaunal former-arthropods evolve lungs and endoskeletons (or a kind of pseudo-endoskeleton), because of whatever specific evolutionary pressures would lead to these evolving, as well as any other adaptations that these former-arthropods would have to evolve along the way.

They evolve from whatever class of arthropods that would be the most reasonable candidate to evolve in this way.

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What would be a reasonable amount of time needed for some arthropod to evolve into megafauna, given the right evolutionary pressures?

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