I actually tried something similar to control a pool pump as part of a solar water heating system. In my case the load was only 15 amps at 120V, and I chose an OMRON G3NE-220T-US, which is an SSR rated for 240VAX at 20 amps. (control voltage=12VDC). In my case the switching was not quite as regular and predictable, but was still frequent enough to raise these same questions for me. especially since the motor would surly draw more on start-up when its mechanical load was higher.
Anyway, I was somewhat dissapointed to find that the heating was a factor, but not so much from the switching as from the normal "running" load condition. Certainly the bare flat metal back of the SSR along with its screw holes on either side was an immediate clue to me that heating (and a heatsync) was to be expected, and it certainly was more heating then I originally expected. Apparently, these SSRs respond pretty much much like a TRIAC , and if you measure the voltage across it under load, you should expect to see an average somewhere between 1 and 2 VAC. Assuming my fluke was giving me a relatively close to RMS value, it became obvious that at 15 amps I was still dissipating between 20 and 30 watts here.
Now how much actual temperature rise you can expect depends of course on many factors. In my case though, the extra 20 watts of heat was a show stopper. Regardless of whether I used a heat sync, the entire control circuit was going to go into a metal box that already had to operate in the heat of a Florida summer. So there was the factor of the heat affecting the reliability of the rest of the circuit, and there was the user product perception factor of a device that would ultimately get pretty warm (if not outright hot) tot he touch.
So just FYI, I finally decided to abandon the SSR in favor of simple OMRON mechanical relay with four 10 amp contacts, (OMRON LY4-12VDC), with all contacts paralelled together and the usual capacitor across the contacts to block arcing from the inductive load. Feels like a step backward in a way, but the mechanical relay has now operated for 8 summer seasons, never gets even mildly hot, and seems a better solution for that reason.