(It's only a month old, so not a complete Revival
)
For Our Universe, I'd say the lore took about a few months. It's nothing too complicated, but there are themes with celestial beings and reincarnations. My Demon Valentine is the same way -- a few months of work, and I already have a few AUs for the work.
Most of my horror work has taken the longest time. I've been working on Beneath our Feet (Above our Heads) for about 10+ years. I only say that because this series is made up of 8-9 comics, all of which I've worked on and off with for about 10 years, give or take. Some of these comics are older than others. However, if I were to count the time I've spent combining these comics into one series and connecting them, I'd say I've spent 3-4 years on that lore.
Most of the lore is based on the type of horror and the impact on the world around the characters. There's subtle world-building, and overall, the presence of the supernatural is unknown until one big cosmic horror event. Then, the ideas of science and religion are shaped. I don't spend a lot of time on this because that's not the focal point of this story, but it is shown in the background.