The future is notoriously hard to predict, but we do know a few things: there is a regular DPJ party president election in September next year (exact date not fixed). The next lower house election must be held no later than the end of August 2013, but is likely to happen at any time during that spring or summer. There will be less than two years — more like a year and a half, and quite possibly less than that — between the regular party president election and the beginning of general election season and campaigning.
Barring a complete disaster1, the DPJ will not want to throw out their regularly elected leader and switch leaders again just a few months before the general election. This means that whoever wins the regular party president election next year is going to be2 their candidate in the general election in 2013.
If Noda is going to win that party president election next September, he will have to show basic electability. He and his cabinet will have to show decent public support numbers, and he must pull the DPJ up from the gutter and at least become more popular than the LDP. And to do that he'll need to show actual progress of some sort — progress on the economy, on the nuclear disaster mess, on rebuilding Tohoku, on the dysfunctional labour market, you name it. Many of these are problems that the central government has little leverage to do anything about, but all of the responsibility to resolve.
And he'll have to show progress in less than a year. With an opposition whose only tactic is to refuse any legislation offered by the government in order to discredit it. Leading a party where half the members want you out in favour of a champion of their own. And with a foreign policy in tatters before he even forms his cabinet, due to his incredibly boneheaded public defence of the war criminals at Yasukuni.
If he doesn't make it, he'll be out in a year. Everybody knows this, and nobody will risk their own career and throw their weight behind him unless he really seems to make progress and will prevail in next years leadership race. In practice he probably has a grace period of a month or less before the consensus verdict on him is in as either a future political powerhouse or a lame duck counting down the days until he follows Kan, Hatoyama, Aso, Fukuda and Abe into political footnotehood.
You may argue that his election is a momentous event in Japanese politics, that his policy ideas of tax increases and a grand coalition with the opposition will be implemented, and have lasting effect on today's problems and on the political climate. That he will stay on to ultimately contest the general election against the LDP. I would not say you are necessarily wrong. I would say the burden on proof is on you.
#1 Never underestimate the DPJ. They seem to have an unlimited capacity for political disaster even where you thought it could not possibly happen.
#2 "will hopefully be" is more realistic. See the previous footnote.