Large institutions can swap baskets of underlying securities for ETF shares that can then be traded on an exchange as part of arbitrage between the price of the basket and the ETF share price. These baskets are called creation units. How often does the creation unit basket for a given ETF typically change? Hourly, daily or yearly?
That depends a lot on the ETF you are talking about. Some of them, for example in corporate debt, have pretty loose standards because so many names are illiquid and consequently there is no single basket that they will or will not take. Well, I suppose the nominal basket would always be taken but no one ever has that.
On the other hand, the liquid equity ETFs require a pretty close match, and therefore the acceptable basket changes monthly (or at whatever the predefined rebalance frequency is).
As Beatrice comments, in most cases the fund manager has wide discretion, and will be trying to maximize the popularity of an ETF by balancing ease of trading creation units versus tracking error.