Winnings from free spins usually arrive as bonus funds with their own wagering requirement, often counted on the winnings alone. Some offers pay spin winnings as cash with no wagering at all, and those offers tend to give fewer spins. A requirement on small winnings can still take a long session to clear, and the balance may run out first. The terms state which of the two applies, usually in the line that describes the spins themselves.
Spins are tied to one slot or a short list of them, chosen by the casino. The qualifying 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.
Free spins expire, often within a day or a few days of being credited, and unused spins simply disappear. Larger offers are frequently released in daily batches, each needing a login or a fresh opt in, so missing a day can mean missing a batch. The winnings from the spins then have their own deadline for wagering. Noting both dates when the spins arrive keeps an offer from lapsing unnoticed in the middle of a busy week.