When should you really start thinking about a NoSQL database?

I've been working with relational databases for ages, and they've been fine for structured data. But lately, I've been thinking about situations where NoSQL's flexible schema and horizontal scaling might be a better choice. Think about storing user content that has different fields for each entry, or dealing with tons of IoT sensor data. What makes you realize a project or feature really needs NoSQL, instead of trying to cram it into SQL? And what NoSQL options do you usually pick for different kinds of problems?