At what stage is it most important to do a product survey?

The most important is to do a product survey during an open Beta test. A lot of iteration will occur during the earlier stage, particularly in the idea stage of it. I haven't tried the website yet, but here you can find people to collaborate with or join your beta test. https://www.polywork.com/

Ideally you want to be trying to collect feedback through a range of methods, including surveys, all the time, however I'd argue that the beta stage is possibly the most valuable. Prior to beta, everything is just hypothetical and what you might collect may not actually translate into real behaviour / usage / issues, whilst at least in the beta stage the users have actually had a chance to use the product.

I'd say the beta stage to see what type of customers actually benefit from your product the most. This is a great blog post https://review.firstround.com/ho...

Realistically, all of the above, ongoing and nonstop. Don't look at the survey as some one off grand event, but instead look at it as an ongoing part of the product cycle. - conduct surveys of your target demographics when you're just coming up with the idea. That way you know you're on the right track, and you have a group of people to hit up when you start building who may feel some investment into your product as their feedback has directly shaped it. - conduct surveys of the potential customers throughout the entire process of building prototypes. Survey/test how easy your interfaces are to understand, how clear your copy is, and how well it converts - collect all the possible data you can (both surveys and actual usage data) throughout the entire beta process - after launch, continue surveying your clients on a regular basis - depending on the type of product you have, as often as 4-6x per year. Always be checking in, making sure that they are still satisfied. If they're not, catch the dissatisfaction early before they quietly churn and you have no idea why. You'll uncover bugs you didn't know were there, get new ideas for features you didn't even think about, and you'll maximise the customer satisfaction when they feel like they've been heard, and the product is evolving into what offers most value to them - while at the same time having the opportunity to directly tell them about new features and create upselling opportunities for yourself.

beta of course.

As others said, I think public beta testing period is the single most important timing to do a product survey. Not only is it the point where you start gathering interested users, but it also is a wonderful opportunity to iterate your product while actively communicating with your beta users, who will likely convert easier in the later stages

Product surveys can be part of the idea validation stage. As early as you can get feedback from users and prospects, the better to adjust your strategy and improve the outcome constantly.