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If it takes a month or two so be it. It will be back just as hard.
I'm guessing 48 hours.
Rumor has it the problem is they were using an outdated version of PHP riddled with security issues that have been long since solved by newer versions of PHP. If so, the real pain and potential down time will not come from restoring from backup, closing a few security holes, and resetting throw away email addresses. It'll be getting such a legacy system to play nice with the newest version of PHP. I've heard some horror stories for similar situations.
How much pain exactly? Don't ask me. I stuck with Perl.