Here's a couple:

Thermogenic, metabolic, and cardiovascular responses to ephedrine and caffeine in man. Astrup A, Toubro S. Int J Obes Relat Metab Disord 1993 Feb;17 Suppl 1:S41-3.

The effect and safety of an ephedrine/caffeine compound compared to ephedrine, caffeine and placebo in obese subjects on an energy restricted diet. A double blind trial. Astrup A, Breum L, Toubro S, Hein P, Quaade F. Int J Obes Relat Metab Disord 1992 Apr;16(4):269-77.

As those studies indicate, the beta-2 sympathetic effects (jitterness, inc. heart rate and bp) subside rather quickly while the beta-3 thermogenic (fat-burning) effects last much longer. I've found this to be the case - been on CKD and using ECA for most of the last 5 years, on a 5/2 day cycle just like CKD itself, periodically taking a month or so off.