I quote from the Tron Wikia:
A Cycle (also known as a TRON Cycle or
TC) is a measurement of time used on
the Grid. Interpreting the dialogue of
TRON: Legacy, a cycle is a fair amount
of time, roughly equivalent to a year.
Kevin Flynn mentions that a millicycle
is about 8 hours. The metric prefix
"milli" denotes one thousandth, which
would mean that a cycle is about 8,000
hours (for comparison an Earthly year,
by the Gregorian calendar, is about
8,760 hours (twenty-four times
three-hundred-and-sixty-five)). But
that measure is speculative and
non-canonical. For more info on
possible time measurment, see Kevin
Flynn's page.
The specially released TRON: Legacy
tie-in magazine guide includes a
timeline of the Grid, explaining that
time moves faster in the system
because its only limit is the speed at
which electrons can move in circuitry.
The guide states that one year in the
real world equals about 50 Cycles in
the Grid, which would mean that Kevin
Flynn was trapped inside the Grid for
roughly 1,000 years from his
viewpoint. This would seem to be
confirmed within the movie itself by
Castor's line that Clu had been trying
to obtain Kevin Flynn's Identity Disc
for about 1,000 cycles.