Three lines at the most. Make it easy to read and understand. Avoid boilerplate remarks, cliche, cute or, lengthy quotes, or ads at the end of your mail. It degrades the importance of your message. It's visual and email pollution or litter.
Ads are the worst.
If you're using one of those email services that puts their ad at the bottom of your email, don't send it. I've had people call me down for it. You know what they are. I won't define them all here. If you're compelled to use a smiley face or wink or any of the others, once is enough.