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

I often cook things that take maybe 7 or 8 seconds. Maybe if they're
really small they take 5 but it's hard to know in advance and I really
dislike most overcooked things. Undercooked, one can just cook it some
more but is two 5-second zappings the same as one 10-second zap?

Or does it take a second (or 2 or 3 or more) to get the waves churning,
so two 5-seconds is more like 9 seconds (or 8 or 6)?

I need to know so that eventually I'll know the proper time for a
particular food, without stopping and checking**.

Has anyone read about this? Or other oscillators?


**This actually raises another question. I'm pretty darn good at
predicting how much time I need to microwave something, and once in a
while I can even remember from experience. (Cocoa from refrigerated
milk is 2 minutes. An eggroll of a given size is 2 minutes 40 seconds,
even though wrapper says 4 minutes.)

But what if part of something much bigger than 10-second food is cooked
enough and I take it to the table and start eating and when I get to
another part, find that it is not cooked enough. By this time all of it
has cooled some. In that case, it's not the microwave warm-up time that
would matter, but am I right that there is still food warm-up time? If
the food is 70^, it might not even start cooking until the part that
cooks reaches, what, let's assume 110. (Or maybe someone has a real
number for a given food)

So if one is eating for 15 minutes, the 120 degree food will have cooled
off to 80 degrees, and if I put it back in, it still has to get back up
to 110 again, before it even starts cooking again, is that right? That
could take 20 or 30 seconds or more, depending on how much food there is
and other details. I dont' mind the extra time, but I would like it if
someone could provide real-life numbers so I could better estimate how
long the 2nd part of the cooking will take.