I like this question since I was asking myself the same question a long time ago. I couldn’t find an answer to it and decided to do it my way. After spending a couple of months trading with a small account size and trading with 0.1 or 0.2 lots, I realised that once I reached a level to trade comfortably with 1 standard lot, it would be enough to make decent monthly or weekly earnings.
So I put this as my goal. Building my account size to a level where I could afford to execute trades with 1 lot.
I gave you my example, however, you should know that this is a pretty subjective question that depends on many factors, such as your trading goal, your trading account size, your preferred instruments for trading etc.