Well it would take at leat 24 years since BA have around 14000 cabin crew, they training capabilities would be bout 50 new entrants every 5 weeks. To keep the operation going you need at least 75% of the 14000 so 10500 new entrants at 50 every 6 weeks you need 1260 weeks to complete this plan with a full speed training department so it is not as easy as you may think.
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Some people here have no idea of the numbers they are talking about, that is because they have never worked for BA in the first instance