Counters used by ifconfig are 32 bit counters, so they can only go as high as about 4.3 GB (technically 2^32 = 4,294,967,296 bytes) before rolling over.  With 10 GB of traffic during a month, that would mean that they roll over a little more than 2 times per month, or about every 2 weeks.

This may be a bit confusing for a few root server administrators that get charged per MB / GB.