At what point would you be willing to sell your profitable side-project?

I read a story about a founder who built an AI writing assistant that went from $0 to $1m ARR in 8 months. He sold it to Jasper AI, one of the other big players.

It all made me think, what's a reasonable point to pull the plug and sell?

Edit: link to article - https://theexitgame.substack.com/p/shortly-the-ai-powered-writing-assistant