The speed of a relay switching operation will help define the lifespan of the relay.  Mechanical relays often have a activation time of 5-10ms, with a deactivation time of 10-20ms.  As a general rule, high-speed switching of mechanical relays will cause physical contact wear.  Solid-state relays are better suited for high-speed switching applications, but they are not immune to the effects of high speed switching.  Users of solid-state relays for high-speed switching applications should monitor the temperature of the solid-state relay.  High-speed switching can cause the temperature of the SSR to rise, particularly when switching high-speed inductive loads.  Inductive loads are particularly brutal for both mechanical and solid-state relays, and in some high-frequency switching applications, custom electronics must be used to properly handle the inductive load.

Solid-State relays are typically available with random turn-or or synchronized turn on.  I prefer to use synchronized turn on, as this type of relay will activate during the zero cross cycle of the AC waveform flowing through the relay.  Random turn-on will activate the relay at a random time, regardless of the position of the AC waveform (and is slightly faster for this reason).  Synchronized relay activation may benefit from having a slightly longer operational life at the expense of activation speed (they are typically a few milliseconds slower).