How far along your project do you have to be to justify getting a business bank account?

At some point you need a business bank account. It's not necessarily about how far along in your project. The main driver is when you need that solution in your business. For some businesses, this will be when they have to accept or pay money for business expenses and they want to have their taxes done cleanly. For other businesses, this will be when you decide to charge for a service or product.

The types of business on WIP are mostly self-run, indie maker businesses. There is a different threshold for getting a bank account than a traditional service business that has to pay for rent, etc.

a good rule of thumb for a US based business is to get the bank account and LLC when you are anticipating that your product will generate actual real income or you will need to track actual business expenses that you do not want hitting your personal income statement. Depending on your personal income, you might just want to have it hit your personal tax statement.. LIke say you make 60k a year from your job, and this side business makes you 5k a year in profit and costs you $500 in expenses. That might mean having a bank account is not worth it and you will claim operating income and expenses on your personal tax statement.

You can go either way in the above situation. But i think my personal approach is to let the income and expense hit your personal account (and personal taxes) until you see that

a) this is long term income generation or the shortterm income is >10% of your yearly income

b) you are committed to the business in some significant way (it moves from side project status to actual business; you find at least some tangible amount of time being spent in the care and feeding of business)
c) you know ahead of time that you are spending some time building the business toward income generation and you've created a company to support that endeavor.