Install After Effects on your C:\ drive.
Put the disk cache on your SSD. Speed matters enormously for this.
how much ?
I'd put the OS & application software on the hard drive. The reason: once launched, there aren't nearly as many read-write instances as there would be for an AE cache. You launch the OS... what, once a day? How about applications.... maybe 3-4 times a day?
On the other hand, when the AE cache is on the SSD, it it reads & writes to it all the time. That's where you want the real speed.
It might be nice to launch in the blink of an eye, but you're taking up valuable -- and pricey -- SSD real estate with OS & applications, things that aren't used all that often in comparison to the AE cache.