If you only read one line of a bonus's terms and conditions, make it the wagering requirement. It determines whether an offer is genuinely valuable or just a shiny number designed to lure you in.
The basic math
A wagering requirement is expressed as a multiplier, like 30x. It means you must place bets totalling thirty times a base amount before withdrawing. Crucially, check what the multiplier applies to: '30x bonus' on a $10 bonus means $300 in bets, but '30x bonus + deposit' on a $10 bonus with a $20 deposit means $900. Same multiplier, triple the cost.
Game weighting
Not every bet counts equally. Slots usually contribute 100% of each bet toward the requirement, while table games like blackjack often contribute only 10%, or nothing at all. If you're a table-game player, a bonus with 10% weighting effectively multiplies the requirement by ten.
Time limits
Bonuses expire. Seven days is common for no deposit offers; some give you as little as 24 hours to use free spins. An otherwise generous offer with a tight window can be worthless if you can't realistically complete the wagering in time.
A quick rule of thumb
For no deposit bonuses: under 20x is excellent, 20-35x is normal, 40x+ means the casino expects to keep your winnings. Combine that with the max cashout to judge an offer's real value. 100 free spins with 60x wagering and a $25 cap is a far worse deal than 25 spins with 20x and a $100 cap.