In heat-exchange and multi boiler espresso machines, it generally takes at least 30 minutes for the machine to be fully warmed up so that the boilers, all piping and tubing, and the group head reach a temperature that would produce consistent and excellent espresso.
Espresso machines with Thermo blocks can take as little as five or ten minutes to warm up, this is because these machines tend to be built using lower-quality material like machines aluminum (versus high-quality stainless steel). The better the temperature stability, the more metal it will have and the longer it will take to achieve an ideal, stable extraction temperature.
A prosumer machine (ex. e61 group head) can be up to temperature in about 20 minutes, but for ideal extraction, it is best to wait about 35 minutes. Commercial-grade espresso machines take longer to warm up, therefore, many users will leave them on 24/7 or put them on outlet appliance timers, which can be programmed to turn the machine on at a certain time or turn the machine off at a certain time.