Using a manual camera app like Halide an iPhone 6 is able to achieve a maximum of 1/2" exposure time, what's the longest an iPhone X can reach?
It is dependent on the Third party app you are using and not the phone itself.
I have several apps that allow me to use shutter speeds from fractions of a second up to 60 seconds and even bulb ( as long as you want ). I have used these apps on my iPhone 6 and my new iPhone 8+.
I use Procam 5 as my main app but i have others. It has a slow shutter mode. ( i am not affiliated with procam, just a user )
Do some research when you are choosing an app to see if you can find out what the manual control options for each app.
EDIT: I asked the pro cam five support staff the question and here is there reply
"Hi , Thanks for getting in touch with us. The longest exposure time supported by the iPhone X hardware is 1/4 sec. Longer exposure time are digitally simulated. Please don’t hesitate to contact us again with additional questions or feedback. Best",