Otherwise, I'd say "Pivot when it's time to pivot".
Sorry for not providing any context. I have been working on my startup, Ziraf for over 6 months now. We're trying to solve fake food reviews problem by getting foodies, chefs and writers to review restaurants (https://zirafapp.com)
We're trying to market the product to students and tourists, but we have not found any product-market fit yet. We face the problem everytime we travel abroad, however, we're not sure whether people actually care about the problem.
I would like to hear from successful founders who found product-market fit before. How did you measure whether there is big enough market for your product?
Here is the link to the app: https://tinyurl.com/zirafapp
PS: I did another Ask HN (https://news.ycombinator.com/item?id=18212201) a while back about being solo technical co-founder before. Sorted that out, we worked closer now and I raised my equities.
Beyond that, I like to start by working backwards from the end goal, at a "back of the envelope" level.
Let's say your goal is to build a company that can IPO. You should be able to look at existing data and figure out some number, like "a company needs X million dollars in ARR to justify an IPO". Then work backwards and project "if we sell this app / subscriptions to this service / whatever, for Y dollars, then we need Z customers to get to IPO range".
You can also work out what your maximum customer acquisition cost can be, given your projected pricing. Then you have a hypothesis to test: We need to show that it's likely that we can sell this app at Y dollars, to Z people, at a CAC of no more than Q". Then you start trying to get people to use your app through various channels (see the book Traction by Gabriel Weinberg for a lot more on that) and then see if you can find a customer base that will support your goal. If you can't, you can iterate on the details of your hypothesis (change the pricing, change customer acquisition channels, etc.). When you "pivot" per-se, is basically when you decide that the market you're looking for, for the app you're planning to build, either isn't there, or can't be accessed in a profitable way, etc.
If you haven't already, I'd suggest reading The Four Steps to the Epiphany by Steve Blank, and the aforementioned Traction books. And/or go through the library of videos from the recent Startup School session. There's a lot of good advice there on dealing with all these issues.
Note also that "get to an IPO" is just a made up example... your goal may be less ambitious. Maybe you just want to build a company with a few million in ARR that can support the founders on a modest lifestyle. The point is just that you need to start from the goal, and see if the business can be built to support the desired outcome.