If Alaska in recent years or even Mt. St. Helens or other volcanoes in recent history is any indication, it won't last weeks on end with no flights. It might disrupt air travel again after it clears but I highly doubt it will last weeks on end.
But it is anyone's guess.
You say it is not unprecedented to last weeks or months. Do you have a reference/link to specific examples?
Might be referring to this volcano, news reports have said it last errupted 1 or 2 hundred years ago and last 1-2 years.