What "casino slot volatility" comes down to is simple: spins are worth less than they sound. Free spins are the most common casino offer and the hardest to value at a glance. Each spin has a fixed stake set by the casino, often the smallest the game allows, and the winnings may land as cash or as a bonus with wagering. The number of spins in the headline says little until the spin value, the selected games and the terms on winnings are known.
A cap on free spin winnings limits how much the spins can turn into, however well they land. Anything above the cap is removed when the bonus converts to cash. Caps are common on spins given without a deposit and can be low enough to make a large hit on the selected slot pointless. The figure is part of the offer terms and matters most on volatile games, where the rare big result is the whole point of playing.
Spins are tied to one slot or a short list of them, chosen by the casino. The selected game decides both the return and the volatility of the offer: spins on a volatile slot mostly pay nothing and occasionally pay well, while spins on a steady one return small amounts more reliably. The game is named in the offer, and its return can be checked in its help screen before the spins are taken.
Often a day or a few days after being credited, and any spins left unused simply disappear from the account.
Many offers cap spin winnings, especially spins given without a deposit, and anything above the cap is removed when the bonus converts to cash.
Larger offers are often split by day, each batch needing a login or fresh opt in, so a missed day can mean a lost batch.
The casino fixes it in the offer terms, often at the smallest stake the game allows, sometimes stating it only as a coin value.
Usually as bonus funds with their own wagering, though some offers pay spin winnings straight into the cash balance.