Kodak high-speed IR comes in two sizes:
35mm and 4x5 sheets. The 35mm
is easier to work with, cheaper, and has a more pronounced grain which
is one of the things that make this film so distinctive.
A good example of
this grain is the stump image
The 4x5 sheet film, like any other large-format film, gives you a much
richer depth of intensity, and much finer grain. The
water temple image is a good example of this.
There are other manufacturers of infrared film, most notably Konica, but these other films do not see as far into the IR as Kodak and I've never tried them.
Anyway, with all that ambiguity, I have a rule of thumb that works pretty well for me: In the cool light of the evening and morning, treat the film as ASA 25. In the warm light of the mid-day, treat the film as ASA 100. In the hot light of an incandescent bulb, treat the film as ASA 400.
The best thing you can do is bracket your exposures a lot. In fact, the "stump" picture actually came from my first test roll when I was trying to determine the exposure to use. You should bracket every exposure +/- one and two stops (for a total of five exposures) if you can.
The other reason to bracket is that different exposures will cause different effects on the film. One exposure may give you a sharper image or more pronounced grain, while a longer exposure may give you more of that "glowing" effect. A good example of this is the trees in the forest photo in which the long exposure brought out the detail in the tree trunks in the foreground while causing the glowing effect in the background.
(BTW, the glowing effect isn't entirely from the infrared light, but from the fact that the film has no "anti-halation" backing. That's an opaque backing on regular film that stops the light that makes it through the emulsion and keeps it from bouncing off the back of the film. Kodak IR has no anti-halation backing and so the light bounces back and forth inside the thickness of the film, causing the highlights to diffuse.
(Of course, some of the effect comes from IR's long wavelength which causes it to "bend" around obstacles that short wavelengths can't pass. That's why the felt light trap in the film cans don't work very well and you have to load the film in a dark room. Black changing bags don't work perfectly either -- you'll be able to see the film leader has been fogged if you change film in a changing bag.
Above all, EXPERIMENT! This is a very difficult film to predict, but the surprises are usually pleasant.
Good luck and have fun.