The lag in subscription times back in the day was always around 8-10 weeks I want to say; even well into the 90's when I was subscribing to about 6-7 titles. I'm assuming you were filling out and mailing in the sub forms from the actual issues...? If so, you were at the mercy of the USPS, plus processing time on DC's end. I remember a few titles being advertised the way you described (get #1-2 together), but it never worked for me either. I subscribed to GL: Mosaic in hopes of getting the free power ring they offered - and I mailed in my coupon in plenty of time - only to have my sub start with #3 as well. Nearly four months after the fact!
But I digress... back to the LoSH!