It's hard to tell what phase we're in without at least 2 or even 3 years shown to be in that phase, since there are occasional blips (short-lived reversals). It now appears that in 2014 we did change to the warm phase after having been in a cool phase since about 1999. About the duration, your sense is correct that it is not a set number of years, and can vary noticeably, as can be seen in the second figure above.