In my side projects, I am influenced by a book I read few years ago: The $100 Startup by Chris Guillebeau. I only do projects that have potential to bring revenue. I use the following milestones to decide whether to keep project alive or not. Once a project is cash flow positive (ramen profit), I only reinvest cash flow.

  $100 + time before project goes live
  $250 + time after launch and until revenue generation
  $1,000 + time after revenue starts coming and until cash flow positive