"How long this will take" is correct and usable in written English. You can use this sentence when asking for an estimate of how much time something will take to complete. For example, you could write, "I'm starting a project, so I'm wondering how long this will take to finish."
"I don't think anyone knows how long this will take".
We don't know how long this will take.
I'm not sure how long this will take.
How long this will take, and what form, I can't hazard a guess".
How long this will take to reach consumers on a large scale is the key question.
How long this will take was not clear, he said, since the company has yet to meet with the agency.
Meanwhile, there's an awful lot of space coming to the big screen in the intervening decade – or how ever long this will take.
Estimate of how long all this will take.
But even he will not make a guess as to how long this mission will take and when it will all be over.
"We have no way of knowing how long this process will take, but it will be a multiple-game suspension," Geiger said.
It is not clear how long this process will take – or whether it will be successful.
Being a terminologist, I care about word choice. Ludwig simply helps me pick the best words for any translation. Five stars!
Maria Pia Montoro
Terminologist and Q/A Analyst @ Translation Centre for the Bodies of the European Union