Many credit cards offer sign up bonuses in the form of points or miles. Now, unless stated explicitly, is there an expiry date for this sign up miles/points?
You have to read the individual agreements for each rewards plan.
It is generally less trendy for points and miles to expire, even the "bonus" ones. But agreements change all the time.
For instance with Chase Sapphire Preferred, customers used to earn "dividends" on the points they accumulated throughout the year, a whopping 7% extra points for accruing points, the agreement here changed, so although in this case, points don't expire, the expected amount of points changed on people.
Conclusion, read the agreements, and agreements change.
For any miles that the credit card company gives you, whether or not they expire will depend on the airline for which they gave you the miles. Once you transfer those miles to your airline's mileage account, then they are out of the credit card company's hands. They are now in your mileage account, and they are treated like any other miles by that airline. So it depends on the airline - some airlines let miles expire after some period of time, and other airlines let you keep miles forever.