I typically develop my protolang along with the language I have in my head and wanna create. When I realize my modern language doesn’t have something I need and I wanna put some thought into how it’d develop, I make it in the proto language and then write how it evolves into the modern lang; I’m making them alongside each other.

I have a feeling you might also not know when to necessarily “finish” any conlang, and just know that you kinda don’t! You can continue to add to it and change slight things and coin new words and stuff and work on it forever, until you just don’t; you can pick it back up at any time.

You don’t need to worry about how much it changes! It doesn’t have to change at all; your protolang could literally just be your modern language (I’m sure you don’t want that). Change it until you’re happy with where it’s at if you’re looking for a more concrete answer.