In general, you usually should not add much delay between star-trail exposures. Waiting for the sensor to cool may reduce heat buildup somewhat, but longer intervals can create visible gaps in the trails.

If your camera supports it, the most direct way to judge this is to check the recorded sensor temperature in the image metadata. On some cameras, maker-note fields can be read with tools such as ExifTool.

Practically, for star trails the usual priority is continuous shooting with little or no gap between frames. If heat/noise becomes a problem, monitor the camera’s temperature if possible and test how your camera behaves over repeated long exposures.