Your vehicle is located as quickly as GPS location is ready and able to transmit back to the main server. This can vary from seconds to minutes, depending on the service you purchase.
Sometimes your vehicle will be out of GPRS coverage but you will still receive these locations when your vehicle comes back into coverage.
There may be times when your device is hidden from GPS satellites, resulting in these locations going amiss. Avoiding prolonged underground parking can alleviate these issues.