I am always careful what I believe since theres plenty of incorrect information even in the peer-reviewed literature. That said,
Disposition of metronidazole in the body is similar for both oral and intravenous dosage forms, with an average elimination half-life in healthy humans of eight hours.
Elimination half-life is often just called "t(1/2)", and the (fairly) constant blood level you refer to is usually called "steady state." At steady state your body level still goes up and down between doses, but reaches about the same peak and the same trough every time.
With a t(1/2) of 8 hours you should hit steady state in just a couple days. In other words, after a couple days the magnitudes of your body level peaks and troughs are within ~15% of the ideal steady state ones.