On Wed, 30 Nov 2016 17:16:05 -0600, Jon Elson wrote:

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.

+1, You are the only one who did not lambast the OP for using the term
'warm up' to mean 'cause the ambient MW intensity to reach its operational
range'.

When I am judging time for extremely small loads (like softening butter
without liquifying it), I allow 4 seconds for my oven.

I find that the hum does not get louder though. Instead, I notice that the
fan speed lowers a bit, presumably because the supply voltage for the fan
is then being loaded down by the power consumption by the magnetron.