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How many days it may take to confirm?
Maybe a few hours to 24 hours? Not entirely known.
Is there a chance it will fail and will go back to the sender?
There is a chance, but it doesn't seem likely based on current mempool.
Transactions are usually dropped after 72 hours give or take.
Is there a chance I could pay by my own a higher fee to this transaction to get confirmed faster?
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Yes, if you can spend the 2.5 BTC now to another address you control with a fee of 142 satoshis per byte.
It is called Child Pays For Parent. A Miner may pick up your new tx faster, confirming both.