src/Core/OAuth/OAuth2/OAuth2AccessToken.php
public function getRefreshTokenValidationPeriodInSeconds(){
      if(isset($this->refreshTokenValidationPeriod) && !empty($this->refreshTokenValidationPeriod))
      {
          return $this->refreshTokenValidationPeriod;
      }else{
        throw new SdkException("The validation period for OAuth 2 refresh Token is not set.");
      }
    }
public function getRefreshTokenValidationPeriodInSeconds(){
      if(isset($this->refreshTokenValidationPeriod) && !empty($this->refreshTokenValidationPeriod))
      {
          return $this->refreshTokenValidationPeriod;
      }else{
        throw new SdkException("The validation period for OAuth 2 refresh Token is not set.");
      }
    }
Return the number of seconds for refresh token
The implementation of Quickbooks Online OAuth 2 always return an 8726400 seconds refresh token. This method is not useful now
@return int