That should be done right at the beginning right after you’ve successfully completed your beta testing and have ironed out the major bugs.

I see many founders worried about customers leaving if they start charging. Newsflash, if they’re not paying then they aren’t your customer.

The only time this may not be applicable is if you have a hybrid model. Squareup.com doesn’t charge immediately but they get paid by facilitating transactions. https://usefulpdf.com doesn’t charge immediately either but it’s partially ad supported.

You should charge immediately and ask for more money than you think the product is worth right now. Most founders underprice by a huge margin.