Thank you for your question Karpit
You require the Access Token with each API call you make to the Translator
The Access Token will expire after a time period of 10 minutes after it is generated. In order to get a new Access Token, you need to make a call to the token service with your client id and client secret.
There is a code sample of how to do this and more information about generating Access Tokens here - http://msdn.microsoft.com/en-us/library/hh454950.aspx
Regards,
Tanvi Surti
Program Manager, Microsoft Translator
Microsoft Translator team - www.microsoft.com/Translator