Can they detain me for that?
Yes
For how long can they keep me in custody/detained before allowing me to leave (either into the USA or back to my home country)?
Indefinitely, but usually less than 48 hours while they work on removing you from the USA as expeditiously as possible. You can google for countless real life examples.
More importantly, can I be arrested for refusing to unlock my devices or provide access to my online accounts such as email or social media?
Detain is technically not the same as arrest however for your case it's just semantics, the results are the same. You will not be arrested and held for weeks because you've committed no crime per se and immigration violations are typically civil offenses however you will be detained pending removal as expeditiously as possible.
TL;DR
For a non-citizen and particularly a visitor, USA immigration port of entry is the wrong place to go about trying to assert some supposed right. You will be the worse to wear for it, and have little to show for your trouble in doing so. At those locations and for visitors in particular, CBP are essentially the judge, jury and executioner.