DOJ strongly suggested they'd be shooting for a 5+ year sentence, which according to the guidelines would have required them to establish losses (stipulating that Swartz had succeeded in his ultimate plan, unimpeded by the countermeasures JSTOR and MIT erected) in the millions of dollars.
Orin Kerr suggests that the likely outcome would have been "anything from probation to a few years", depending on where the judge landed on the financial damages.
Popehat has a really good take (as usual) on the circumstances faced by Aaron Swartz, but it's not as specific as Kerr's, which is the most detailed breakdown I've seen available on what actually happened with the case:
