You should definitely have a significant amount of total liquidity... combination of short term investments, cash, committed bank line of credit, other liquid stuff, etc.
I don't know what kind of business this is, so I'll ignore the specific guesses in any direction and keep it general.
The most important thing is to be able to stay liquid. The easiest way is to project some simple and possible scenarios within the realm of possibilities, and how prepared you want to be. So, in the recent downturn from 2007-2010 (somewhere in that period depending on what your business is exactly), how did the company do? How long was the slowdown in sales/profits/cash flow, and what did you have to do to stay solvent and keep things running?
In a future downturn, how much would sales have to decrease in order for you to start losing money to keep the doors open? How long could the business stay liquid if sales dropped 10/20/30%? How volatile is your business/industry overall?
Answering some things like this will help you as you plan how "risk averse" you are and what you would want to be prepared to handle.
You asked about a line of credit. Is it secured? Is it guaranteed? Can they cancel it for any reason? It would suck if the economy went south, you needed the cash, and the bank cancelled the line.
You didn't put in a bunch of info, so I'm speculating a lot here. Just some things I brainstormed as I started thinking about liquidity. I may have gotten a little complicated than necessary. I work in a Fortune 10 treasury dept, and work with liquidity issues all the time, so I see how all the crap hits the fan on at least a monthly basis.
Good luck :)