how much boat before I need a captain?

I asked the owner of a 70' Offshore pilot house about this. His underwriter required a captain aboard for the first season. That is five or six months in the Great Lakes. Another requirement was that the owner get licensed (not six pack, as I recall). After the first season he was the the owner/operator and said he had no problems. I don't know if he was limited to the lakes.

I was in a similar boat so to speak (bad pun there, sorry!) when I moved from a 330 Sundancer to the 550 Sedan Bridge we have now. I had the 330 for over 10 years and had no claims so when I moved up the insurance company just required that I get a check ride with a captain and that he write a letter saying I was qualified to operate the 550.

My wife and I are on it together 99% of the time though I have single-handed it, and we have no problems. She's a great deck hand and that makes my job a lot easier.

I would say your insurance company's requirements would depend on how long you've been with them and your claim history and total length of boating experience.

The insurance company is the one who will require the captain and for how long. Let me stress however that this doesn't mean that you can't run your own boat. I've been working for a gentleman for several years who's completely capable of running his boat. I taught him several years back with the intention of him going on his own. He still won't go out without me. It's a fairly easy gig for me, but it seems that every day something comes up where I earn my salary. There's just no way to buy experience, but you can rent it.
The key to enjoying a boat is to make it as hassle and stress free as possible. Otherwise she'll spend her life as a dock queen. 50' can be a lot of boat. It's not just about driving it across the bay. It's about reading the waters, the dockmaster squeezing you into that space meant for a 46', the mechanical problems that can arise, and a million other whoops situations that can easily cost more than you'd have paid a captain to be by your side for 5 years. I'm working a boat right now that has some things that leave me scratching my head after more than 50 years on the water. So I turn to the yard manager who knows these boat well, and he says 'I'll have to ask someone else and let you know'. Boating is one of those things where the more you know, the more enjoyable it is, and there's always more to learn.

So long story short, don't fight the insurance company on their recommendations. A good captain will be like cruising with a knowledgeable friend on board, and one that minds his own business. And I'd almost bet that every day you'll see a situation that makes it well worth whatever you pay him.

There is the "what can you handle" and the "what makes sense from a safety standpoint.
With enough experience, you can handle 40-50 feet, but why would you want to take the chance. In certain conditions (and they come up quickly and unpredictably), the experience factor goes out the window, and no matter what, a second (or more) pair of hands, becomes invaluable and indispensable. Engine room fire, leak, mechanical breakdown, fowled anchor preventing retrieval per electric winch...the list goes on.
I prefer to be conservative, safe, and less stressed, leading to a more enjoyable experience. Unanticipated emergencies come up anyhow, dont invite them.
$0.02 from 35 years of boating experience in the Chesapeake Bay and Lake Michigan.