I think it depends at what stage you are. If you are at the beginning then the answer is in the (conservative) estimate of how long it will take you to start getting some real money in. This can be very tricky to get right so be very conservative.

If you are already at a stage where you have some cash inflow from sales then I would say 6 months reserves is what you need. If you see that you have more than enough cash in the bank to last you 6 months without any sales then I would say expand.