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How long can it take for this attack to happen? Any ideas.

You mean how long the attacker wants to hold the secondary chain hidden? Nobody knows. The actual takeover in terms of hashing power can happen instantly.

That's what I wanted to know. Instant take over rings some alarm bells for me. Next question though. Is it technologically possible to prevent it from happening?

From a technical point of view, no. Technically speaking, this is a Bitcoin fork, which uses Bitcoin hashing power. So, technically speaking, it can happen.