  Long-term instability in multi-planet exosystems is a crucial consideration
when confirming putative candidates, analyzing exoplanet populations,
constraining the age of exosystems, and identifying the sources of white dwarf
pollution. Two planets which are Hill stable are separated by a wide-enough
distance to ensure that they will never collide. However, Hill stable planetary
systems may eventually manifest Lagrange instability when the outer planet
escapes or the inner planet collides with the star. We show empirically that
for two nearly coplanar Hill stable planets with eccentricities less than about
0.3, instability can manifest itself only after a time corresponding to X
initial orbits of the inner planet, where log_{10}(X) is of the order of 5.2
mu^{-0.18} and mu is the planet-star mass ratio measured in (Jupiter mass/Solar
mass). This relation applies to any type of equal-mass secondaries, and
suggests that two low-eccentricity Hill stable terrestrial-mass or smaller-mass
planets should be Lagrange stable throughout the main sequence lifetime of any
white dwarf progenitor. However, Hill stable giant planets are not guaranteed
to be Lagrange stable, particularly within a few tens of percent beyond the
critical Hill separation. Our scaling represents a useful "rule of thumb" for
planetary population syntheses or individual systems for which performing
detailed long-term integrations is unfeasible.
