Sam Flynn: "Long time"
Kevin Flynn: "You have no idea..."
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 metrix 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 [1] 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.
(According to mathematical calculations done by Tron community fans, a more precise number has been achieved using the Tron: Legacy tie in magazine guide as a basis. Going on the basis that 50 cycles on the grid is one year in real time, breaking down the numbers goes as thusly. One second RT = 0.000001585489599 cycles. One minute RT = 0.00009512937594 cycles. One hour RT = 0.0057077625564 cycles. One day RT = 0.1369863013536 cycles. One year = 49.999999994064 cycles. Rounded up it is 50 cycles. Now, to break it down further using those number as a basis, it comes out that one cycle is 7.3 days RT. And if you multiply 7.3 by 50 cycles, you get 365, the number of days in a year.)