Sleep cycle length is a highly individual thing, normal variation is between 60 and 120 minutes, but it tends to be fairly constant for a normal person. Recently I have been using a little gizmo called Zeo Mobile
http://www.myzeo.com/ that uses EEG from a headband to determine sleep state. As it turns out, for me my sleep cycle length actually changes over the course of the night, starting out about 110 minutes at the beginning, and ending around 75 minutes near the end. This is also very consistent from night to night, within a few minutes each way. Sleep cycles also tend to be different for napping then they do for night time sleeping. Most consistent nappers enter REM immediately after falling asleep for a nap, while most enter deep sleep first when going to bed for the night.
The gizmo takes a bit of getting used to for sleeping, but it only took a couple days for it to feel normal to sleep with it. It has also given me some interesting results, for instance I tend to get about 10 minutes more deep sleep on nights that I train. This makes sense given that deep sleep is most restorative to the body and is when a bunch of hormones like HGH are released. I am also about to start an experiment to see how ZMA effects my sleep cycles too. Well worth the $100 it cost IMHO.