You get a write amp of 1 until the drive is filled the first time. After that, it's a function of
1) how full the drive is (from the drive's point of view—this is why TRIM was invented)
2) the over provisioning factor
3) usage patterns, such as how much static data there is
4) how good the SSD's algorithms are
5) other (should be) minor factors, such as wear leveling
Source: I used to be a SSD architect.