Time-frame within which a bitcoin price, converted from other currency, is valid — what is recommended?

On my website I’m setting bitcoin payment by myself. The prices are in USD/GBP/JPY and a few other major currencies. But the payments should be in bitcoins.

When I’m generating an invoice for a client and converting a USD price to bitcoins, what do you think will be wise time-period within which a bitcoin price is valid? Approximately. Meaning, it’s very unlikely to go up or down and at the same time isn’t too short. 1 or 2 hours? 5 hours? Or 10 minutes? Is there an official recommendation about that?

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