This is an example of an electronic diagram of a toaster timer. The manufacturers use various assemblies often based on a frequency oscillator and a timer (The MC14541BCP, datasheet provided here). The setting of the time is done with a potentiometer (P1 here).

Each device is slightly different, so we won't go into the detail of the electronics here.

The modified tip at the bottom of the diagram is worth noting. This prolongs the longevity of the contacts of the bipolar switch (see the chapter on the origin of toasters' breakdowns) Necessary tools: 1 triac BT138 + 1 resistance 4,7k

Source: http://www.rvq.fr/tech/g-pain.php