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It differs from person to person. One may buy 2 times in one day with less amount of money while another could buy 2 times a month with a larger amount of money. But in order to call it DCA, it needs to be done after a fixed amount of time with a fixed amount of money.
So how often one should buy in this market depends on what he has been doing in the past. He needs to continue the same strategy if he wants to continue doing DCA.
And yes, DCA could be profitable but as always with Bitcoin investment, you need to wait and find the right time to sell in order to book your profit.
Lastly, the time for selling! It also depends on the person. If you have a higher target, you should wait till the target is reached. If not, sell little by little. While keep on buying in the lower market as well.