I like the stages described by TMI, but it seems they might run into an issue when you consider how time affects one's performance.

If I stick to 15min/day, it feels as if I progress through the stages relatively quickly. But then, when I start doing longer sessions e.g. 45mins/day, I regress in my technique & this even affects my technique when I go back to 15min/day.

Did I just transition too quickly to longer sessions? Is it because I did longer sessions on days that I'm tired?
What are some good guiding principles in choosing meditation length?