Long Exposure Timer is a stand-alone Apple Watch app, designed for long exposure photographers by a long exposure photographer.
Meter the scene with your filters off, or the ISO raised, set the initial metered time and the number of stops to add in the app. The adjusted time will be calculated, and if it is over 3 seconds you can use the timer. Although it will work down to 3 seconds, it is probably best to use the camera’s shutter speed up to its maximum time for the best accuracy.
Long Exposure timer will request permission for notifications, this is only used for notification when the timers complete, and will not be used for other purposes.
There are a full compliment of complications, as well, which allow you to see the remaining time on the current timer, as well as launching the app.