When should you invest in a pbn?

I am currently working on my first real project by myself that isn't related to the marketing agency that I work for. I am trying to expand my knowledge on all this stuff and make some money while doing so. In a perfect world, I want to build a PBN network then sell it to the marketing agency, however, for the time being I just want to build it out for my own personal endeavor. At what point do I invest money to buy them? I am just getting started with this separate project and am about to finish setting up my website by the end of the day today.

1 month in I would say. Drip feed 1-2 links from your PBN to your money site per month.
My personal opinion: use PBN links to inner pages especially the ones with high search volume and medium competition. Once you rank for a medium comp keyword you'll start getting natural links.

When you have so much money to spend, and when you tired your Links doesnt stick and cant target so many urls.

One good exp/aged domain cost a lot, $2k++

Building a personal pbn is a great choice for specific websites if you're looking for long term.
If your focus is short term investing in a good pbn network won't be fruitful.
Rather look for quality pbn services that rent links on monthly/yearly basis.

Building a quality pbn involves good investment of time, money, skills, & efforts.
Filtering out quality domains itself is a task that requires diligence.