Sorting out the ecosystem will give you answers.
This setting has a fairly large ecosystem problem that needs addressing. Working it out will give additional details that will likely shed some light (pun not intended) on some things you're probably wondering about.
In our world, the sun gives life. All life, except for chemosynthetics living in deep-sea vents and the like, ultimately gets its energy from sunlight. The light makes plants grow, and everything else is supported off plant biomass.
But twilight is so much dimmer. Here is a rough intensity chart. Note even a cloudy day is close to two orders of magnitude brighter. Very roughly this translates to two orders of magnitude less energy for living things.
The problem is this corresponds (very roughly, mind) to two fewer trophic levels, since the rule of thumb is there's an order of magnitude loss going from level to level (i.e. herbivores get 10% of the usable energy out of the plants they eat).
In other words, if your world has nothing but unending constant twilight, that's not enough energy for the kind of energy dense plants that form the base of an ecosystem supporting animals on top of it. It might support plant life, but not things like fruit trees or flowers. And none of it would be fast growing.
To avoid an empty world, something else must be going on
A world empty of everything but slow growing and stilted plants isn't very interesting to read about. You're going to need something else that makes plants grow. Seasons will likely be marked off, and 'years' pass, in relation to whatever that turns out to be. Here are a few possibilities.
- Rivers of ambrosia
These rivers have an unknown source (the sap of Yggdrasil, thermosynthetic life forms living atop a mountain that's an active volcano, gushing out of the ground in geysers, whatever you want) and they fertilize the soil.
To be agrarian, your people have to use ambrosia to fertilize the land. They mark time by the Barrel. Giant barrels, big enough to make the land fertile for a season of crops. Different epochs will be marked by different sources of ambrosia; as large amounts of time pass, this geyser dries up, another one takes its place, and thus Yosemite 217 is followed by Velikan 1.
- Manna from heaven
Like the old story, periodically the land is fertilized by literal manna raining down. But this doesn't happen often; your society marks the passage of time by Breadfall - each time manna falls marks what we consider something like a year.
- Supernatural creatures
Phoenixes turn out to be migratory birds that fly in flocks. Every once in awhile, they'll pack up and move. Wherever they roam, their very presence brings life to all around them. When one dies and is reborn, a circle about 1.4 miles in diameter will bear fruit for what we would call ten years.
Or perhaps there are clouds of frost-mites, insects bearing supernatural cold from the seemingly-endless glaciers. They move through an area like locusts, but after they've been through, mysteriously, the plants grow back twice as verdant.
The "seasons" are then based around the lifecycle and migration patterns and so on of the creatures forming the backbone of the ecosystem.