vnStat is a console-based network traffic monitor that uses the network interface statistics provided by the kernel as information source. This means that vnStat won't actually be sniffing any traffic and also ensures light use of system resources regardless of network traffic rate.
By default, traffic statistics are stored on a five minute level for the last 48 hours, on a hourly level for the last 4 days, on a daily level for the last 2 full months and on a yearly level forever. The data retention durations are fully user configurable. Total seen traffic and a top days listing is also provided. Optional png image output is available in systems with the gd library installed.
See the official webpage for additional details and output examples.