How much BTC is safe on Exchanges?

I've been into Bitcoin for almost five years now. Back in the day, I picked some up for just a few hundred bucks and ended up taking profits in the thousands. Honestly, I never wanted to sell, but life had other plans and I had no choice at the time.

Thankfully, I've bounced back since then and have been slowly stacking again over the past six months. While I don't have the same buying power as before, I'm still glad to be back in the game, even if I'm holding about a tenth of what I used to.

That said, I’ve always kept my BTC on exchanges just because it makes buying super convenient for me. But I do realize that coins on an exchange are kind of like fiat, just numbers on a screen, not truly yours.

So here’s my question: at what point does keeping BTC on an exchange become too risky? Is there a certain limit or rule of thumb you follow before moving funds off to self-custody?