Can cover anytime really, retail of course can do so partials/small fills, bigger traders/funds larger fills, downside is the spread (electronic trading only, rather than market quotations) can sometimes get a "bad" fill if spread too large, but I doubt it makes that much of a difference if you really need "out".
To answer your question, anyone can buy/sell/cover anytime as long as enough liquidity in pre-M/post-M, usually people do not bc of the lack of liquidity on both sides (bid/ask) which can greatly affect the pps if share size too large. I think most wait till the open.
News 
Market Data 
Discover 
AI
