The standard way to try to estimate seconds without a
timepiece is saying "one thousand" between the
numbers.
However this is awkward and inaccurate due to the fact
that you're depending on rhythmically recited syllables in
a
cadence to approximate a certain amount of time for
numbers that
have a varying number of syllables. "One
one
thousand" has 4, "eleven one thousand" has 6, "thirteen
one
thousand" has 5, "one hundred one one thousand" has 7
etc.

The current way to deal with this is to say the number as
fast as you need to fit into the cadence. This is clumsy
and
annoying and when you get into the hundreds it's pretty
useless.

I've tried this other method and it works. You use 4 little
sentences to count up to your number and you change it
as
necessary to keep an even 4 syllables through most
numbers. It's not as complicated as it sounds. The
sentences are: "and then a" as in "1 and then a 2 and
then a
3 and then a...". Next sentence is "and a" for the longer
numbers as in: "13 and a 14 and a 15 and a 16". Longer
numbers just get "and" as in: "101 and 102 and 103 and
104".

There are a couple of bumps where the odd extra or
lesser
syllable number pops up but it's pretty easy to
compensate
for.

"9 and then a 10 and then 11 and a 12 and then a 13 and
a
14 and a 15..." That sounds tricky but it's not. Give it a
try.

Now kdf will breathlessly point out (and I of course thank
him for his helpful research) dance instructors
use "one and a two and a..." regularly to set up a timing,
but as far as I know, nobody has used a varying number
of
words to fit various numbers into a standard 4 syllable
cadence.

If you're counting off seconds beyond about 30, it won't be
particularly accurate no matter the method (accumulative
errors). So worrying about "hundreds" is probably overkill.
On the other hand, 99% of people carry a smartphone which
has a stopwatch in it...