Does anybody remember if they said how long Annie was in hiding during her hunger games, before the arena flooded?

I don't believe they ever said how long exactly. But I do know that after her tribute partner was beheaded, she went insane. She left to go into hiding, and after a while (or maybe not that long) there was an earthquake (whether its real or capitol made, I can't remember), that flooded the entire arena. Annie only won because she was the best swimmer.
But I could share my opinion on your question, I believe that they (meaning the capitol) wouldn't let her hide for that long. They want action, not some nutcase sitting in a cave.
Hope I helped :)

When Annie's tribute partner was beheaded, Annie went completely insane. She went into hiding, and after a while, (they don't say exactly) there was an earthquake and it flooded.

So again, it doesn't say exactly, but it shouldn't be too long.