When does my skill tier start to decay?
The higher your Rank, the sooner your will start to decay. Just play regularly and you'll be fine, and if you're at risk, your progress bar will turn bright red to warn you that you're at risk of decay.
What is the skill tier decay rate?
We're not getting into that because we may tune the decay rate dynamically as we learn more and receive player feedback.
Does my skill tier decay affect my Rank?
Your skill tier points are going down as you decay.
Will skill tier decay affect my Rank in casual and Brawl modes?
Skill Tier decay is for Ranked only.