Here are some examples of side projects that turned into highly successful companies:

1. Airbnb: Co-founders Brian Chesky, Joe Gebbia, and Nathan Blecharczyk started Airbnb as a side project while they were working on other startups. They created the platform to help make ends meet during a difficult financial period. Airbnb went full-time in 2008, and today it's a global leader in the vacation rental market.

2. Dropbox: Drew Houston created Dropbox as a side project while studying at MIT. It started as a simple solution to the problem of forgetting USB drives. Houston realized the potential of the platform and went full-time in 2007. Dropbox is now a leading cloud storage company, valued at over $12 billion.

3. Uber: Travis Kalanick and Garrett Camp originally created the ridesharing platform as a side project while working on StumbleUpon, a social discovery network. Uber's success led to both founders devoting their full attention to the company. With a market cap of over $60 billion, Uber has transformed the transportation industry.

4. YouTube: Steve Chen, Chad Hurley, and Jawed Karim initially created YouTube in 2005 as a dating website before realizing its potential. The three founders went full-time in 2006, and Google later acquired the company in 2006 for $1.6 billion.

5. Buffer: Joel Gascoigne started Buffer as a personal productivity tool to help manage his social media accounts. It gained popularity, and Gascoigne quit his job in 2011 to focus on the company full-time. Buffer is now a globally renowned social media management platform.

Regarding how founders decide it's time to go full-time, it typically depends on a few factors. Achieving significant product-market fit, securing significant traction, and generating revenue are some indicators. It's also essential that the founders have sufficient financial runway to cover living expenses and grow the business. Other factors could include market opportunity, team support, and personal circumstances; it should, however, be noted that going full-time as a founder shouldn't entirely depend on these indicators alone. There might be significant considerations or personal circumstances that result in a founder going full-time that differ from one founder to the next. In general, it's essential to thoroughly analyze all aspects of the business before making the decision to go full-time.