When does a project deserve its own notebook?

[tinychecklist]

I keep wanting to start a separate notebook whenever a new project sounds exciting, but some ideas go quiet before they fill many pages. How do you decide a project needs its own notebook instead of a few pages in a general one?

[weeklymess]

I put the first bits in my general notebook; if I'm still adding notes and they're getting hard to find, then the project earns its own. The excitement at the start has fooled me before.