When should a startup file for a patent?

Each jurisdiction is different in terms of its “first to disclose”, “first to file” and “first to invent” rules. First to file and first to invent - Wikipedia

You should file a provisional patent application as soon as possible (to establish your patent’s priority date) if you are filing your patent application in the US because USPTO follows a “first inventor to file” rule. First Inventor to File (FITF) Resources

Vietnam Patent 

I’ll recommend the following course of action to help you file a provisional patent application as quickly as possible:

a) Conduct a through prior art search as soon as possible to make sure that your idea is novel, non-obvious to others in your field, and has utility in real life.

b) Draft your first independent claim. Draft any additional independent claims if you’ve more than one. Don’t waste your time on drafting any dependent claims at this stage.

c) Focus on writing as many possible embodiments you can think of for implementing not only your independent claim(s) that you have drafted but also for all possible dependent claims (that you’ve not drafted). Refer to 2164-The Enablement Requirement to guide you in writing your embodiments.

d) While drafting your embodiments, draw as many diagrams as possible to show your invention and its implementation. Follow Patent Drawing Rules: Everything You Need to Know. No need to use any fancy tools for doing your diagrams. Make your drawings by hand for now to save some time.

e) Provide a brief definition of any special terms that you are using in your patent claims and embodiments.

f) Draft a paragraph to serve as abstract. No need to write any paragraphs covering background of the invention or summary of the invention. Providing a list of prior art is also not needed at this stage.

You are now ready to file a provisional patent application. File it without any delay to establish a priority date. You’ll have a full year to file your non-provisional patent after this date. During this one year time, also called a “pendency period”, patent process is pending till you file your non-provisional patent application.