Blogger accounts and Blog*Spot addresses do not expire.
If you setup a Custom Domain, things are a bit more complicated. If you don't pay the periodic DNS hosting, or the name registration, fees, the domain will expire, and eventually go out of service. Google for "Black Viper", if you want an example of what the readers go through, when that happens.
Your blog will continue on BlogSpot, but won't be accessible. Both "www.yourblog.blogspot.com" and "www.yourblog.blogspot.com" will be forwarded to "yourdomain.com". With no DNS entry to forward "yourdomain.com" to "ghs.google.com", all 4 aliases ("www.yourblog.blogspot.com", "yourblog.blogspot.com", "www.yourdomain.com", and "yourdomain.com"), will be discontinued.
You'll need to have your blog switched back to publishing to BlogSpot, when DNS service is discontinued. Many Bloggers will do this, for one reason or another. Maybe you can give a friend backup administrative authority over the blog, so it can be switched back to BlogSpot, when you are gone. At least with that strategy, "yourblog.blogspot.com" can continue forever.