"how long then" is a grammatically correct and commonly used phrase in written English.
It is typically used to ask about the duration or length of time before something happens or is expected to happen. Example: "I'm going to the store, but I'll be back soon. How long then until dinner is ready?".
How long, then, could Israel sustain the fight?
How long then before the grandees of the EU dismiss these agreements as scraps of paper?
How long, then, must the police wait after they have released a suspect?
How long, then, will you put off thinking yourself worthy of the highest improvements and follow the distinctions of reason?
And then another thought caught Mr. Beresford breathless: how long, then, had the man in the light hat been following him?
She would calculate how long, then add to that the time it might take him to shop for supper (figuring that a man alone would shop every day).
How long it then would take to make and package millions of doses is unpredictable, she said, but should take at least six additional months.
Life surrounded by nature, how long ago? then the introduction of industry became the dominance of industry, and technology seems to be stealing, mind and soul without a whisper.
Natural language processing systems figure out who billed what and for how long — and then that data is run through a machine learning system that flags outliers.
If one of those content offers is a SlideRocket online presentation, the data on who viewed the presentation and for how long can then be transferred back to Marketo for use in scoring the value of the lead.
You have to physically go to the hospital or some office, where you wait (with no real predictability for how long), and then the nurse probably takes you in and checks your vitals.
