It depends on the rules
Often, in games where the house has a rule against mid shoe entry, if you sit out a hand you must wait for the shuffle.
If there is no rule against mid shoe entry you can sit out and come back whenever you wish.
Since the average blackjack player is an ignorant and superstitious creature, going in and out of hands will get you some grief often. Go out of a hand and everyone loses or come back in and everyone loses you will probably hear some comments. Of course, if everyone wins no one will say anything. Remeber it is your money so you do whatever you want. Blackjack, contrary to what these players will tell you, is not a team sport but is every man for himself (unless you play on a team that shares profit and loss).
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