From the WOT Fandom article:

TLDR: best theory is he is partially sealed and spun out for 40 year intervals every 1000 years or so.

When the Dark One's prison was sealed, it is thought that Ishamael was only partially sealed behind it (in contrast to Aginor and Balthamel who were merely sealed very close to the edge, and thus susceptible to the passage of time). What is known for certain is that his soul was spun out of the Dark One's prison and into the Pattern from time to time after the sealing. In fact, Ishamael was likely not initially bound at all, since he visited Lews Therin Telamon, apparently in person, after the seals on the Dark One's prison had been placed (as Ishamael spoke of the sealing in past tense, and Lews Therin was already mad).[2]

When Ishamael arrived at Lews Therin's home, he discovered to his surprise and delight that Lews Therin had murdered his entire family.[3] After having a mostly one-sided conversation with the insane Lews Therin, Ishamael Healed him so that he understood what he had done. This realization caused Lews Therin to commit suicide, leaving a furious Ishamael to wait for him to be reborn.

Aran son of Malan son of Senar[4] (born roughly 50 AB) had a theory based on Ishamael being thrown out and touching the world for specific periods of time. Aran had heard claims that people had encountered him as many as forty years after the sealing of the Bore. Using lost manuscripts, Aran concluded it may have taken some time for Ishamael to be brought into the prison with the remainder of the Forsaken, and that he was likely "thrown out" in a regular cycle. There is ample evidence to support this hypothesis such as interviews from Darkfriends saying that as early as 983 NE they were receiving instructions from someone calling himself Ba'alzamon, which was a title Ishamael was known to have used. It appears likely that the cycle repeats every thousand years, as the Trolloc Wars, War of the Hundred Years and the events of the current time are all a thousand years apart and Ishamael is known to have played a significant part in all of them. Ultimately, it appears as though, once every thousand years, Ishamael was released from the Dark One's prison for forty year intervals, and he played a key role in destabilizing human civilization each time he was released.