It really depends on a lot of factors (i.e. type of business you are in, how much cash intensive it is ...).
As a general rule though, I would advice you to BOOTSTRAP AS LONG AS IT IS POSSIBLE.
Bootstrapping has a lot of advantages, nonetheless that you get in lean mode and make the best of each possible opportunity (or else you are out). Bootstrapping, while building a set of CORE METRICS around your business, will help you realize the real value and potential of what you are building.
As CORE METRICS (revenue, users, ARPU, LT, LTV, churn...) improve, the cost of financing will decrease. So you will be able to get funding on better terms.
Also following a path similar to this, will give you a much better understanding on WHAT YOU NEED FUNDING FOR (besides getting a nicer office). Metrics will tell you and investors will know.
Can you be bootstrapping for ever?. Probably yes (if this is what you want and how do you want your business to evolve), although most probably not advisable. When the metrics are right and you have a fire burning (as clearly seen in the METRICS), I would say it is time to get some petrol and make it much bigger.
Happy to chat about my experience.
At times it could be a bit tricky to understand the right time to seek investment. The necessity of fund could be understood only by you, not anyone external to the business. If you feel you have tried and tested the limits of bootstrapping, then funding could be an option.
However, do ensure that you've done product/service validation before seeking investment. An investor may invest in your team, but sans validation of product/service you can end up in more turbulent water.
Secondly, too much money could be dangerous either. Necessity is the mother of all inventions. Too much money could help you do more of same thing, but you may not be able to brainstorm the better ways of doing much better things.
Limited capital could also be helpful in building small and iterating fast. You know, Titanic never returned. But small lifeboats do return. Fail small, Fail early, Fail fast is the mantra to succeed.
So, the right stage and time to get funded is when you have tested your limits to bootstrap, and after you have validated your critical process and product.
In my more than a decade experience working with startups and entrepreneurs, I have helped plethora of them with "Hand-Holding" and investment planning. Try to bootstrap in a right way. Feel free to reach out for any further information that you may need. I am just a clarity away. All the best!!