Does access token obtained with OAuth has expiration date?

Our application uses OAuth to get user’s access token. We noticed that after a while tokes are revoked and API calls fails with unauthorized errors. An example:

```json

{

  "type": "error.list",

  "request_id": "0003hg8qe7evlq01dqcg",

  "errors": [{ "code": "unauthorized", "message": "Access Token Invalid" }]

}

```

May I ask if access tokens obtained through OAuth have expiration date? If there a way to debug why token is invalid now?