In the Book of Causation (Nidaanavagga) VII The Great Subchapter 61, the Commentary by Buddhaghosa - see below- gives the figure of hundred thousand kotis of cittas arising and ceasing in a fingersnap.
This is to help people who, reading the sutta, might imagine that one citta last a whole day or a whole night.
Obviously, even without any insight, one can see while, say watching TV, sound objects and visible objects and thinking rapidly alternate, and so there are different cittas arising that then immediately cease. In fact it is so fast that I have read some Buddhists wonder if it is the same citta seeing and hearing (and even thinking??)at the same time!
(1) Uninstructed (1) p. 595 Samyutta Nikaya Vol 1 (translated by Bodhi)
But that which is called 'mind' and 'mentality' and consciousness'
arises as one thing and ceases as another by day and by night. Just
as a monkey roaming through a forest grabs hold of one branch, lets
that go and grabs another, then lets that go and grabs still
another, so too that which is called 'mind' and 'mentality'
and 'consciousness' arises as one thing and ceases as another by day
and by night. [note 157]
[note 157: Spk: 'By day and by night (rattiyaa ca divasassa ca):
This is a genitive in the locative sense, i.e., during the night and
during the day. Arises as one thing and ceases as another (annadeva
uppajjati, anna.m nirujjhati): The meaning is that (the mind) that
arises and ceases during the day is other than (the mind) that
arises and ceases during the night. The statement should not be
taken to mean that one thing arises and some thing altogether
different, which had not arisen, ceases. "Day and night" is said by
way of continuity, taking a continuity of lesser duration than the
previous one (i.e. the one stated for the body). But one citta is
not able to endure for a whole day or a whole night. Even in the
time of a fingersnap many hundred thousand kotis of cittas arise and
cease (1 koti=10 million).