Without compensation the maximum error can be approximately ±10°C, where the biggest deviations from the ideal value occur at the temperature extremes. Two-point calibration and compensation gives the best results, but still, one-point calibration can result in satisfactory temperature readings if the temperatures of interest lie close to the calibration point. The following data come from measurements made on the Atmel ATtiny45.
The Figure 1 and Figure 2 above show the ADC conversion error of an arbitrary device at 1MHz and at 8MHz CPU frequency. Each point in the graph represents the average of thousand consecutive temperature measurements. In both graphs, the line with squares represents readings in ADC Noise Reduction Mode, while the line with diamonds represents the readings in active mode. As seen, the measurement errors at a high CPU frequency are larger if the measurements are carried out in active mode compared to ADC Noise Reduction Mode. At a low CPU frequency the differences are small.