micky wrote:

How long does it take a microwave oven to warm up?

The heater in the magnetron tube takes a couple seconds to warm to the point
where it will emit electrons. You can hear it easily on the old transformer
microwaves, the fan starts and it begins to hum, then a couple seconds later
the hum gets much louder. That louder hum is when the magnetron tube starts
conducting.

Newer microwaves with switching power supplies may delay turning on the HV
until the heater has warmed up, and they may not have that transformer hum,
so it may be harder to tell when the RF comes on.

Jon