What's your tax residency? Some countries (eg. Germany) tax mining as regular income -- based on the exchange rate at the time of mining payout -- regardless of holding time.
I would really want to know about this, does that mean that if at the point of payout bitcoin is $15000 and Op have to hold till next year and price has increased to say $20000 when he is converting to fiat. Will he be subjected to tax at $15000 or $20000 or its the margin that would be paid as tax and if the price falls to say $10000, would he be entitled to a refund?
Usually you have to pay income tax in advance based on your estimated profit for the year. If you overpay, you'll get a refund. If you pay too little, you'll have to pay additional taxes and they will estimate a higher profit for the following year (unless you give them a good reason to assume otherwise, eg. when you expect to shut down your mining operation).
To clarify what profit means in this case: Say you receive two mining payouts during the year. 1.0 BTC while BTC is at EUR 15,000,- and 0.5 BTC while BTC is at EUR 10,000,-. Your taxable profits are now EUR 20,000,- regardless of BTC being at EUR 5,000,- or EUR 50,000,- at the end of the year and regardless of when and at which price you sell. So if at the end of the day you owe them, say EUR 5,000,- in taxes, that's all they want. They don't care whether you had to sell 0.1 BTC or 1.0 BTC. Note that no additional tax is applied to this sale -- or any future sale of your remaining BTC after that.
The bad news is, if BTC crashed to EUR 1,000,- and you didn't cash your taxes out before that, you'd have to sell all your BTC and would still owe them money.
The good news is, you get to deduct the cost of your mining equipment and electricity from your taxable profits.
