How long does it take to rewrite from scratch a web application with 200 pages?

This question came up during a meeting.

The assertion was:

1. we have all the requirements because we have a working application

2. we'll split frontend and backend dev

3. it takes in average 1/2 day to build a web page (as per industry standard?)

I'm not very comfortable with this kind of question and analysis, specially when asserted by an influent and experienced board member (which on the other side has only recent functional knowledge of our application).

I'm also probably biased because I was personally involved in building technically the application during 6-7 years.

It looks to me as a very aggressive estimation, but I hardly know what to answer. Analysing all the 200 pages, estimate each one of them, etc. will take at least a month. Plus at the same time, a complete new UX/UI is required from the new app.

The application is also multitenant and multilanguage. and must scale well to 1thousand tenants at least.

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What people answer to managers in this kind of situations?