bitcoin reorg question if a block takes 10 minutes to be mined

does it need 21 minutes to be reorged?

if the reorg started immediately after the block was mined

because 100% was mining and 51.5% will reorg

how long does it take to reorg 5000 block ? 70 days?

is 70 days of reorging a huge thing that would make the people stop believing in BTC ?

all of my questions and availability here is because i am selling my business for 5300 BTC and i dont need to be F&amp;&amp;&amp;ed with reorging later if i will not use them

this was also my point in the last questions for creating custom wallet offline
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 000000000000000000b5c...

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.