The first “law” of TPRS is, keep all input comprehensible. Part of this skill is going s-l-o-w-l-y in class. How slowly? You’d be surprised. If you feel like you are explaining to a slow two-year-old, you still might be going too quickly.
The right speed? My estimate is about 110 words per minute.
Get a stopwatch and say the following sentence aloud. Time yourself:
“I am going to school tomorrow with my friends John and Mary.”
This should take you ten seconds to say. If so, the speed is around 110 wpm. Yes, it’s slow. But that is the speed of comprehensible input.
Don’t believe me? Check this: in a fascinating article I just ran across at Ben’s, sent in by Latin master Robert Harrell, audiologist Ray Hull explains the speed of language.
The basic facts:
— Average 5-to-7 year olds process their native language at about 120 words per minute.
— Average high school students process native language at 140-145 wpm
— Average adults speak at 170 wpm.
Fred Rogers (“Mister Rogers”) trained himself to speak at a rate of 124 words per minute – and kept children spellbound. It’s instructive to watch just five minutes of “Mr Rogers”…that is almost as slow as we should sound in a second-language classroom.
Dr. Hull says, “Anybody who works with children will save a great deal of time if they will simply speak at a rate children can comprehend,” [then] gives an example from school: “So when an algebra teacher is speaking at 160 or 180 words per minute and is introducing a new math concept … that is a problem.”
Article here: http://dyslexia.wordpress.com/2008/08/28/help-students-hear-your-words-speak-slower-says-audiology-professor/
t.p.r.s. q&a 
Not quite. That sentence is 12 words long. Ten seconds is 1/6 of a minute. 12 x 6 = 72. If it takes you ten seconds to say 12 words, you’re actually speaking at 72 words per minute, not 110. If you want to speak at 110 words per minute, it should take you about 6.5 seconds to say that sentence. Still, I found this article informative and will be using it to adjust my speech.