Refresh Token Expiration time Google calendar

Refresh Token Expiration time Google calendar

Can any one please tell me the expiration time for Refresh token
generation through OAuth2. Actually it usually returns 2 parameter access
token and refresh token. We used refresh token in order to generate a new
access toke if access token get expired. But Google Calendar Version3, i
am using refresh token in order to call the calendar API. But here i am
getting a problem that token get expires. So can any one please suggest me
what can i do when token get expires. According to me there is no
expiration time for refresh token.