How many times have you written to the same cell on your SSD? Current NAND chips have a write cycle limitation of somewhere around 10^6. Apparently a Taiwanese firm got it to 10^9 in 2012, but I'm guessing that hasn't arrived in production yet. So they have raised the write cycle limit by somewhere between 1000 and 1,0000,000. If you're wearing out your SSD once a year (I'll assume you are using it for swap or something), they've increased it to 1000 yrs. That's pretty much the minimum. So "almost infinite" seems reasonable.
The fastest DDR3 is PC3-17000 which has a write speed of 17000MB/s peak. Or roughly 16GB/s. http://en.wikipedia.org/wiki/DDR3_SDRAM
Assuming that you re-write your entire RAM every second, that's 10^12 seconds or about 32000 years. https://www.google.com/search?q=10%5E12%20seconds&rct=j
But if you're writing to your RAM at a 10 microsecond interval (say you're doing something like a firewall or HFT) and you keep re-using the same few megs of RAM then instead of 32000 years it's more like 4 months.
So it's fantastic for "durable" storage, but definitely not as good as SRAM or DRAM in general. The lifetime of SRAM and DRAM are both going to be based on the lifetime of the silicon which is roughly related to the temperature of the chip and the thermal migration of dopants. Hotter means more energy for dopants to move, but generally measured in many years or decades.
