Honestly, it depends on the moderator who handles the flag. No, to truly be honest--it depends on the whim of the moderator at the time he/she sees the flag who decides to handle the flag.
There are no hard rules about this. It is all subjective. I'd say the one thing you can be guaranteed of is that questions with an accepted answer or that are older than a year won't get migrated.
If a question has an accepted answer, there is pretty much no point in migrating it. Usually, in these cases, it is acceptable to flag as off-topic (if you cannot vote to do so). The question can be closed as such.
Old questions are also generally not worth migrating. If the question is from ought-nine or before, I'd suggest you vote to close/flag as off-topic, and edit the question to add the following to the top:
This question exists because it has
historical significance, but it is not
considered a good, on-topic question
for this site, so please do not use it
as evidence that you can ask similar
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