I have measured that in the past week. One of my websites has more than 500.000 user accounts. I picked the users who were not on the site in the last week, because they have already seen the news on the site, and got some 384.000+ distinct e-mail addresses. I needed to contact them regarding an important issue about the website. The e-mail was composed like this:

Hello,

you are reading this mail because you are a member of [mysite link].

Issue explained and link with [call to action]

Regards,

Your webmaster
[mysite link]

Links where not plain text, but special URLs I used to track the clicks.

I sent the e-mail slowely over a 7 day period. I wanted to track weekends and working days as well.

And here are the stats:

• 384408 different e-mail addresses
• 13966 bounced back (3.6% bounce rate)
• 9345 clicks (yielding 2.5% conversion rate)

Hot spots in the e-mail message:

• 23% clicks on link in the first sentence
• 65% clicks on call-to-action
• 12% clicks on website link in signature

Here are the weekday stats:

• Monday 13%
• Tuesday 16%
• Wednesday 18%
• Thursday 18%
• Friday 15%
• Saturday 9%
• Sunday 11%