Two common answers are

• It depends
• 6 month into your monthly burn rate

Lets dig deeper.

4 months from starting is a reasonable time to assume that you are making a monthly profit. So you need to keep 4 months recurring plus the fixed expenses to start.

Fixed costs:

• Cost of incorporating a legal entity: A1
• Cost of hardware(Computers, printers) to start: A2
• 1 year of insurance for founders and employees: A3
• Deposit/Down payment for office rental: A4
• Miscellaneous one time costs (Buying a domain etc) A5

Recurring costs:

• (Lets assume two people) Salary B1/Month: 2 * 4 * B1
• Electricity, Janitorial, AC etc: 4 * B2
• Cost of web services used(Project management/CRM): 4 * B3
• Cost of servers: 4 * B4

Put these values in a spreadsheet and get the amount of money you should get before you start. In general between 5-9 months of recurring expenses are a fair estimate.