As soon as you're fed up with listening only to your your own feedback. You are
not representative of your product's averaged userbase. Stop avoiding showing your product to outsiders and get their opinion sooner rather than later.
Validation of your product, let alone the workflow, colours, fonts, aesthetics, etc is crucial. If your product is promising enough, you'll retain the early adopters as evangelists going forward. If it's not good, then you should be able to get some great feedback - sometimes that is "stop what you're doing and find a different idea, this one sucks".
Any feedback is good feedback. Start now.