ECA stack is great if your diet is in check and your cardio. lcgator is right with taking only one pill to asses how it effects you at first. If you handle the Ephedrine ok and after about a week to get used to the stimulatory effects, then 2 times a day is good for most people at the 25mg mark. Im not convinced about the Aspirin part of the ECA stack personaly but you def should not need 325 mg. I would suggest 2 or 3 of the 81mg pills. So..
25mg Ephedrine, 200mg Caffiene, and 160-240 mg Aspirin two times per day. Dont use the ECA stack close to bedtime, more like morning and mid afternoon.
Oh and as for how long you run it, I have heard that Only stimulatory effects of Ephedrine wear off somewhat quickly but thermogenic effects are good for 12-15 weeks. Now i have never ran and ECA stack longer than 6 weeks without a break of 2 - 3 weeks off, but i can say personally that it was effective through the 6 weeks i ran it. You should for sure be able to run it for 3-6 weeks before taking time off IMO.