If an invention has already reached market, can the technology be patented by the inventor(s)? Is there a threshold of time post-hoc or just not possible?
In the U.S. there is a one year "grace period" to file from the date of first public exposure. In most other jurisdictions there is no such concept. In the U.S. there are several events that can trigger the start that year and it can be complex to determine when/if it was triggered, but being available on the open market is clearly one of them.
In most of the world there is zero grace period. Public exposure can make it impossible to obtain patents in many other countries so that it is usually important to file before a public exposure.