While I was using Avast my system SSD drive got average 1-1,5 GB writes per day. That is with all the necessary settings for a system SSD drive: TRIM supported and on, index, prefetch, superfetch off, pagefile, browser cache, temp on other drive etc. SSDLife application showed that the brand new drive will last for a year. After two months usage I have installed another antivirus application (this was a month ago) for test, and since then my writes went down to average 200-300 MB per day. You should check this out. As I remember Avast caches some results for faster checking, maybe that part worth a look.