At what times do you suggest doing press releases for startups
Journalists have rather simple rules what makes an information newsworthy:
A "lot of people" often just means a large part of the audience of a certain media.
Basically any event that fits one or more of the above criteria is newsworthy for some media and you can make a press release about it.
Hope that helps.
Press releases in themselves are not very valuable. Most new media doesn't care anymore. If you blast a press release on the wire, it most likely won't be picked up by anyone.
A better approach is to target very specific media and make sure you have some kind of relationship with them. You can also learn about exclusives and see how the business of making news works. That helps in understanding how journalists and bloggers pick the stories they cover.
To answer your question: communicate when you have news. First product launch is definitely a major news (look at the TC50 and DEMO events if you doubt that the press cares about first launch). Investors is also news worthy, but with a more professional target (your endusers don't really care that you got funding).
Again, the key test is whether it's newsworthy. If it is, communicate.