First you need to define "reorg". Under the definition I'll use, the minimum reorg is two blocks. The definition would be:
"A reorginization occurs when a block is broadcast with a history that is incompatible with the previously work-heaviest** known blockchain, under the same consensus rules"
Say a block is mined onto the height 593000 block, with hash
000000000000000000a11... so that apparently, block 593001 has been found, and many miners are now mining based off of this hash.
Now imagine that someone broadcasts block 593002 with hash
000000000000000000c84...but upon examining block 593001, you see that the hash is
A reorg has just taken place. Someone mined block 593001, and without broadcasting it (or perhaps after broadcasting it too late), mined 593002 on top of it. Now any transactions in the block with hash
000000000000000000a11... have not been confirmed (one of which was the original miner sending the block reward transaction to themselves), since the (valid) block that contains them has been orphaned.
As far as "how long", a reorg is relative to the observer, so one could technically take place before you observe the original chain at all.
** essentially, the longest chain, but there are exceptions where a "reorg" could be a shorter chain in terms of number of blocks, when a shorter chain contains "more" proof of work. This is outside the scope of this answer.