IMO the right time to get a guage is when you change something from stock. A gauge can help monitor the modifaction and ensure that it is working correctly. For example, if you wanted to fit an oil cooler. An oil temp gauge would be a good idea so you can get an idea of the need for the cooler. After the cooler is installed you can use the gauge to make sure it is operating correctly.

There are some gauges that are good even on a stock motor though, and these days car manufactures are putting less and less instrumentation into new vehicles. Our old EA series falcon S wagon even had factory oil pressure and volt gauges! Oil pressure is something which is nice to have on any car really.