Phrase Usage – Is ‘How Long Will You Be’ Correct?

Sure. There are lots of ways you could ask, and that's one of them:

• Will it take long?
• Will it take a long time?
• Will it take a long time to finish?
• When do you think it'll be done?
• When do you think it might be done?
• Could you tell me when you think it might be done?
• Do you have any idea how long it might take?

As usual, you can insert meaningless hedge words like might to soften the question, and you can make it more polite by using less direct language such as could you instead of will. What level of politeness or directness is appropriate is, of course, something you'll have to determine by context.

And context can change everything. For example, if you're dropping off a document to get copies printed up, you might ask when you can come back to pick them up. The following question isn't literally about when the printing will finish or how long it will take, but it would nonetheless be a perfectly natural way of asking:

• When should I come back?

But speaking devoid of context, your way is fine.

"raison d'etre" doesn't fit very well here. While the literal translation into English is "reason to be," when English speakers start using French phrases, it's usually because we're trying to express something more than the literal translation. Otherwise we'd just speak English.

It's most commonly used to refer to someone's primary purpose in life: the thing that matters most to them.

"Music was Beethoven's raison d'etre."

"Her children are her raison d'etre."

"When the war ended, the soldier lost his raison d'etre."

While it can be used in other contexts, in a lot of cases it will just come across as gratuitous French.

Here are some phrases that I think express what you want:

They do not question the reasoning behind the process, they just follow it.

They do not question the purpose of the process, they just follow it.