First questions are (1) are you cashflow positive on your planned/forecast sales and (2) how much confidence do you have in your forecasted sales.
I think for revenue-free companies or companies where revenue is highly unpredictable, I like having many quarters of life based on my burn rate and my existing cash balance.
The issue is, IMHO, as you get bigger this gets harder to do and becomes an impractical constraint. For example, consider a $10M/quarter in revenues business -- should it have $60M in the bank? Probably not -- unless doing $0 is a realistic possibility for a few quarters in a row is a realistic possibility.
So my short answer is, as you get revenues the better question is "how many quarters of life" should I have in various scenarios. To me, if you can model a big, improbably (but non-zero) negative surprise, make cuts in response, and end up on a new, lowered run-rate without running out of cash, then you're OK.
So switching from quarters-of-burn thinking to scenario modeling. My 2 cents.