Most electronics tend to last longer when they stay on rather than being turned off and on. Heating and cooling of components is more harmful than a constant temperature with a bit of fluctuation within the mfg. tolerances. If the modem was properly designed it should be able to handle periods of ramping up much better than being shut off and becoming cold and then turned back on going from cold to hot.
Like Doug said, just lay a pancake fan and 90F thermoswitch on the modem case would be better if your worried.
Ron ~ Wildblue Select Pak since 01-2006, Anik F2 Beam 26, service through NRTC, Dell XPS, 2Gig Ram, XP Pro, Router DD-WRT- WRT54GL, Laptop, Dell XPS - Windows 7