The reason you lump multiple card applications together, and submit them all on the same day, is that the credit card companies then don't see what the "competition" - ie, the other card companies - are doing when you're applying. Plus, if you are applying for more than one card from the same company on the same day, then credit inquiries for that company will be consildated into just one inquiry. So you get only one "pull" for two (or more?) applications. You credit score gets dinged by 4 to 5 points for each pull, so consolidating pulls is advantageous. Example: my wife got targeted for the Amex Gold biz card for 75K MR points after only $500 spend. I also wrangled a referral for her for the new Amex SPG 30K points card. I put the apps thru on the same day - in fact, within 1 second of each other, using two browsers - and she got approved for both, yet only got one inquiry on her credit report.

The reason you wait 90 days (or more) between churns is that many credit card companies will view an inquiry as not recent if it's more than 90 days old. They look less favorably on your credit worthiness with many recent inquiries. Waiting more than 90 days makes the inquiries not quite "recent". This varies, of course.

It helps your credit score if you have long standing accounts. So you should keep at least one or two cards for the long term. Your best bet is to identify a card that would be a good every day spend card, that you will use after you've reached all your new card spend thresholds, and plan to keep that card for a long time. This is the card that will get you good points value independent of the sign up bonus. Many think the SPG card gives great value for everyday spend, and I agree. I also need a Delta Amex Reserve to help me achieve airline status, and a Marriott card because of my hotel needs, but that's just my situation. You need to decide what works best for you. My oldest card is now 14 years old. I have another that is 12 years old. All the other cards I keep until shortly before the annual fee is due, then I cancel. That way my "average age of accounts" (which is a stat that the credit companies look at) is maximized. But if I'm not going to be using a card for ongoing reward benefits - in other words, if I only signed up for it to get the big sign up bonus, then I cancel at the 11 month mark. That gives me room for the next churn.

I churn 3 - 6 cards every 90 days. I keep telling myself I've got all the points I need, and there's no good offers left, and I won't be churning for a good 6 months....but then something new always comes along that I canNOT resist (Hello BA 100K miles!).

Hope this info gives you a useful starting point to develop a strategy. Thefrugaltravelguy wrote a book that helps - all profits go to charity - you might want to check that out. And FT is so huge with info, it can be daunting, but if you keep reading, you'll soak up info.