Trim sessions by time, not count

[cowwoc]

Version 3.5.9 saves the last 5 auto-saved sessions. In the past I sometimes mistakenly opened two windows of Chrome. On of them contained 100+ tabs and the other ~10 tabs. When closing Chrome I mistakenly closed the 100+ tabs first so when Chrome was subsequently reopened it only restored the window containing the 10 tabs.

I am afraid that with 5 auto-saved sessions I might not notice my mistake until the backup I am interested in gets wiped out.

Ideally, I would like to maintain ~7 days worth of sessions, regardless of the count.