If there are out of town guests, sending invites 8 weeks before is a good idea.
The Save The Date card *should* be an indication that you will be invited to the wedding, but the way some brides and weddings are these days I personally wouldn't buy plane tickets without having received the invite (or confirming directly with the bride). Because what if they realized their 325 person list was too much on their budget and decided to scale back? I wouldn't chance it.
OP, if you need to get plane tickets I would call the bride to confirm the plans and that you will still be receiving an invite. Or, wait until you get the invite and if it's too late to get a decent rate then you just miss the wedding. How close you are to the couple getting married will likely dictate which you choose.
Dh's cousin is getting married in mid-August. We were told months ago by dh's mom that her SIL (the groom's mom) informed her that we would be invited but that it was adults only. Dh's aunt passed this message along so we could all start looking for sitters since usually MIL or SIL watches our kids and other SILs kids.
Last Sunday at a family picnic I asked how plans for a shower were coming along, and dh's aunt looked at me oddly. You didn't get an invite she said? Um, no. So she asked my SILs, and they hadn't either. She was SHOCKED, since the invites had been out for weeks and she DID put us on the list. I felt bad, because I didn't really care was just making conversation. So the next day all 3 of us got e-mailed shower invites from the bride's sister. It's less than 2 weeks away, and we've all already got plans.
Then yesterday we had the other side of the family over and MIL asked for our wedding invite to show someone. Um, we never got one! LOL So here we go again! I think the bride and groom are in over their heads. Or maybe the right hand doesn't know what the left hand is doing. Either way, no big deal but in this case if it was out of state there is NO WAY I would buy plane tickets!