How long between Singapore tourist visas?

To the best of my knowledge (I lived in Singapore for ~8 years and moderated an expat forum there), Singapore does not publish any official guidelines for repeat visits. However, it's a very small country with limited opportunities for tourism. If you spent the entire length of your previous social visit pass (three months?) in Singapore and want to return as a tourist for another long period of time, Immigration's bells start ringing. But if you spent only a few days in the country and are returning for another few days, that will most likely be perfectly fine.

You did not say why you want to return to Singapore, but if your interest is more than just tourism, the solution is simple at least in theory: get an actual visa. Work visa if you're working, long-term visit pass if you're visiting friends or family, etc.