The classic on this is an article by Thomas Warfield (developer of Pretty Good Solitaire), tucked away in the archives of the ASPects newsletter by the Association Of Shareware Professionals. I’ve forgotten the title, unfortunately. He had his app embed in the buy now link information that contained what day of the trial period the user was on when they bought it (and other information, like which nag screen button they’d clicked on, so he could test which nag screens worked best). Once he’d had lots of sales, he could chart which trial days people bought his software.
But you may not need to go to that effort. For a consumer download product, a lot of sales will occur in the first “golden hour” of trying the software (so make that first hour of use perfect). The majority of sales will probably be on day 1 & day 2 of the trial, with fewer sales each day… and one last spike of sales (small or large) on the day the trial period ends.
Also, there’s a lot of people who buy software without trying it first.
Depends on the product. If you just have a nag screen, lots of people will keep using it for free, but maybe they’ll tell their friends. That worked for Sublime Text, but it seems lots of their sales come from businesses who don’t want to risk having unlicensed copies in use.
It’s probably best to block some features eventually. Many consumers just won’t pay unless it’s the only way to continue using the software. I’m sure some older developers here have stories of being on day 427 of their 30 day Paint Shop Pro 4 trial…