First off, given the context of the scene I'm pretty sure Thor was just being facetious with the "half a million years" comment.
Secondly I don't think anything has been specified with regards to Hela's age, however in 'Thor: The Dark World' Loki infers that Asgardians live for about 5000 years.
Obviously she's older than Thor and as such can't be any younger than 1500, so with a little guesswork we can narrow it down a little more.
From the first 'Thor' movie we know that Thor just recently came of age and was expected to take the throne in Odin's stead. If we assume for a second that this is typical of Asgardian Princes, then we may further presume that this was roughly Odin's age when he succeeded Bor, married and produced an heir.
So with all that in mind we can safely say that Hela is between about 3500 and 2500.
Furthermore, given that Odin was still warring with the Ice Giants when Thor and Loki were infants, one could presume that Odin's new peace initiative and Hela's failed rebellion were fairly recent, which would have made her about 500 to 2000 years old at the time.
Split the difference and make her 3000 and that also makes her 1500 when she made a play for the throne, which seems to perfectly line up with the "coming of age" idea mentioned above.
Indeed if she was of an age where she expected to take over the throne and Odin refused her, (much like Loki) it could further inform why she took such drastic action against her own father.