That's one of those questions that doesn't have a right answer.
You can keep too much cash, and bleed it all away before ever making a profit.
You can keep too little cash, and hit terminal problems quickly.
What you need to do is set it up so that you'll be making a profit as soon as RI climbs a little, if not before. My most recent start-up, I got 11 small-medium planes, got down to my last $1000 scheduling them, and then went into the red to pay staff. A little negative isn't an issue if you planned it in advance, it just stops you getting planes, getting slots, renewing leases.
If your start-up will be profitable in 2 months' time, you can go right down to zero or less. If your start-up won't be profitable in 2 months, then keeping cash on hand won't actually help.