CubeSat at 700 km could stay in orbit for over a century, but drag makes it a gamble
Calculating atmospheric drag on satellites for a Cubesat [pdf]
A 2010 Lawrence Livermore report by Wim de Vries presents a simple method to estimate orbital lifetime for CubeSats, combining an exponential atmosphere model with solar and geomagnetic indices. For a 3U CubeSat at 700 km, lifetimes range from about 10 to 57 years depending on cross-sectional area, with the smallest area exceeding 100 years. The model is calibrated against ISO standards and validated using the Genesat 1 CubeSat.
Re-entry is assumed once the object's altitude has dropped below 180 km. At these altitudes all but the most massive of objects de-orbit within a few hours, thereby serving as a convenient end-point to the calculation.