  Markarian 421 (Mrk 421) is one the brightest and closest (z=0.031) blazars
known (de Vaucouleurs et al 1991). It is also one of the fastest varying TeV
$\gamma$-ray sources, with a flaring activity on time scales as short as tens
of minutes. The activity of Mrk 421 at different frequencies may reflect the
radiation mechanisms involved. Tluczykont et al. (2007) estimated the TeV
activity of Mrk 421 through calculating the fraction of time spent in flaring
states at TeV energies (TeV duty cycle) by using data from several imaging
atmospheric Cherenkov telescopes (IACTs). Since IACT observations are biased
towards high flux states they overestimated the TeV duty cycle of Mrk 421. Here
we propose an alternative approach to calculate the TeV duty cycle of Mrk 421
that takes advantage of the continuous monitoring of the source by the Milagro
experiment, a water Cherenkov detector sensitive to primary $\gamma$-rays
between 100 GeV and 100 TeV. We present our estimation of the TeV - duty cycle
and study its robustness.
