What is the normal duration of human sexual intercourse?
First, "premature ejaculation" is a matter of definition. Here is an authoritative source. From the American Urological Association's news site, a 2008 notice:

'The ISSM [Int'l Soc. for Sexual Med.] has defined premature ejaculation as “a male sexual dysfunction characterized by ejaculation which always or nearly always occurs prior to or within about one minute of vaginal penetration; and, inability to delay ejaculation on all or nearly all vaginal penetrations; and, negative personal consequences, such as distress, bother, frustration and/or the avoidance of sexual intimacy.”'

The main author of the ISSM paper was Sharlip. Sharlip ID, Hellstrom WJG, Broderick GA: The ISSM definition of premature ejaculation: a contemporary, evidence-based definition. J Urol, suppl., 2008; 179: 340, abstract 988.

If this is the threshold for PE then it is not a stretch to say that almost anything else is within normal limits. If "typical latency is 4-8 minutes" then 1.5 minutes would be near the low extreme of normal. 

I think the issue of stress is at least as important as duration. If something isn't bothersome it never becomes a medical issue.

A nice study involving 491 men is reviewed here. The mean IELT (intravaginal ejaculatory latency time) is 5.4 minutes. As you can see from the graph on the site, most of the results are contained in the first few minutes with a tail of longer times to the right. 

The original source for that information is MC Waldinger et al. A multinational population survey on intravaginal ejaculation latency time. Journal of Sexual Medicine 2005 2: 492-497. This also addresses your question about the timing of measurement--it's post-entry. 

An expert in this area could update my information but it's a recent average so I don't think things have changed much.