Yeah I thought it was pretty amazing that we're just finding out about such large-scale--albeit diffuse--features of the galaxy! Who knows what else we're missing (well like all that unaccounted-for "dark matter" for instance)?
Even 50,000 years would be something of an eye-blink in astronomic time, but the NASA article (this one: http://www.nasa.gov/mission_pages/GLAST ... cture.html
) said the bubbles may be "only a few million years old"--although it didn't say why they thought that. According to this: http://en.wikipedia.org/wiki/Galactic_c ... population
, the Milky Way is thought to undergo a large starburst every 500 million years or so (we're supposed to be due for one in 200 million years) when the accumulating gas and dust around the core reaches critical density, so since the last should have been about 300 million years ago, the bubbles might not be from that, but that's not to say that there wouldn't be intermittent smaller outbursts on a somewhat regular basis as Sgr A* swallows individual stars, nebulae, and stellar black holes that drift too close.
And since the black hole is always there spinning, whether or not it's active, I'd think a large-scale magnetic field that could shape the bubbles could be a fairly permanent feature, even if its strength may vary depending the hole's activity level--but that's all total conjecture by my totally unqualified self!