This is heavily dependent on your overall strategy and risk tolerance, and also what you're holding.
For me, I try to limit it to to 5-7% each core holding (Foundational) and then smaller for more speculative/growth options. This is why you'll see Berkshire at 6% of my portfolio and Aritzia at 2%.
In addition to this, if you have broad based ETFs, they can be much more. Prime example, when I owned XAW, which is an ex-Canada broad based ETF, it was 50% of my portfolio. If you have an S&P 500 ETF, it might be 10,20,30% etc.
In general, I follow the rule of no more than 5-7% for core holdings and 3-4% for smaller conviction holdings. I will let my winners win though. If a core holding goes to 10% I wouldn't be worried. I might consider rebalancing at years end.
It's all up to the individual!