  The Milagro experiment was a TeV gamma-ray observatory designed to
continuously monitor the overhead sky in the 0.1-100 TeV energy range. It
operated from 2000 and 2008 and was characterized by a large field of view
($\sim$ 2 sr) and a high duty cycle ($\geq$ 90$\%$). Here we report on the
long-term monitoring of the blazar Mrk 421 with Milagro over the period from
September 21, 2005 to March 15, 2008. We present a study of the TeV variability
of the source and provide upper limits for the measured flux for different time
scales, ranging from one week up to one year.
