Every credit card issuer has its own reward program terms. For example, gives 1.25 miles per dollar spent and rather big one-time bonus. To see how many miles you need for one free flight, you just need to add two zeros to the cost of the flight. Say you booked a $232 flight, you will need 23,200 miles to cover it. This card has no annual fee and if you are not carrying a balance, all miles you earn will definitely cover the expenses.
Compare other credit cards which allow you to earn bonus miles or bonus points. Those cards have different rewards programs and ways you can redeem your bonuses.