Will my contacts see my last seen in WhatsApp even though I didn't use it for a month now? How will it looks like?
Yes, if you have enabled your last seen then it will show to your contacts whenever you were logged-in last time.
From WhatsApp FAQ:
Last Seen tell you the last time your contacts were using WhatsApp.
Last Seen refers to the last time the contact used WhatsApp. Through our privacy settings, you have the option to control who can see your Last Seen.
It looks like this: