to me, meditation is an exercise in strengthening the brain's ability to focus and tolerate stressors, as well as developing a window into awareness.
as such, like any exercise, you see results as soon as you begin to exceed your current capacity. tho you might feel a little 'sore' or tired, this is a good thing, as it indicates you are making progress~! even noticing that you are irritable is a way forward.
i encourage you, and all meditators, to persist, making it a part of daily life.
.... follow the llama ....