How long does underwriting take on a fha home loan Help?!

Ok we started looking for a house because we got approved for a fha loan. In the beginning of April we got approved for a house that we wanted. Our original closing date was May 27 but something when wrong with a appraisal and we had to get trees cut then the seller lost there appraisal and had to get another one for them and long story short it was every little thing after another. We got the final appraisal done on Friday and they just got all the papers to finalize this morning. Our lend said they just have to go through the underwriting process. It has taken so long that the seller has threatened to back out and put the house back on the market my question was how long does underwriting usually take? Because the seller said if we don't close by Friday they are backing out.
Added (1). Think again tomorrow is Wednesday because today is Tuesday. And no the seller has to get a appraisal done when they put their house on market. We got the trees cut a while ago when they told us to. There was other issues with the other appraisal like light on a tree little petty stuff. And after a reappraisal to check if everything was fixed that go done Friday and papers came in at the end of yesterday and today our lender said that they are in line for underwriting.

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