Does “Sundowning” ever stop for people with dementia or does it become less frequent?

I found that elimination of all possible stress in the afternoon helped. Even phones calls could set my Dad off.

he enjoyed coloring books...and it helped him to stay calm.

but, it only worked in the evening hours. Late at night he would be awake and causing no end of trouble...every night.
i never found anything that could help, and the use of an RX for sleep was totally impossible because of a lifetime of liver damage from RX drugs

The symptoms of Sundowning can stop abruptly, fade, or continue over time. There is no clear way to predict this behavior.  

A method of treatment for sundowning is through lifestyle changes that can help to distract the person and help them to sleep. These include:

Enjoying relaxing activities such as gardening together in the late afternoon
Gentle exercise like walking
Giving the person a favourite and familiar hobby to do at difficult times of the day
Considering whether something that’s happened during the day could be upsetting them
Minimising naps and getting enough daylight
Avoiding caffeine and alcohol in the evening

There is light therapy - that can help too.