My opinion:
For banks that are sensitive to new credit: 90 days minimum, 6 months is better, 1 year is best.
For banks that don't care: 30 days is probably all you really need, and many times you don't really need to wait at all. There are people who have loaded up on a dozen different store cards and various credit cards in the same month without issue.
Like others already said, it depends on the bank, your history with that bank, your credit profile, etc. But I think those guidelines are roughly accurate for most people.
Total Loan Balance: $43k / $65k
Total SL: $78kAlso keep in mind that there are really TWO goals whenever you apply for a card.
1) To be approved (obviously)
2) to get good terms (APR, CL, etc.)
A lot of people don't really think about #2 when they apply, they're mostly just concerned with getting approved at all. So while you *may* get approved 90 days after you got your last new card, you might have gotten better terms if you held out for 6 months or a year.
I see a lot of people on this forum who have tons of really good prime credit cards with huge limits, and then they complain when some new credit card they got only came with a $500 CL - and I suspect in many cases, it's because the bank approved them because of their good scores and credit history, but kept the limit low because the huge amount of recent credit made them nervous.