According to the simile given in Sāsapasutta kappa can be approximated.
Suppose there was an iron city, a league long, a league wide, and a league high, full of mustard seeds pressed into balls.
“Seyyathāpi, bhikkhu, āyasaṃ nagaraṃ yojanaṃ āyāmena yojanaṃ vitthārena yojanaṃ ubbedhena, puṇṇaṃ sāsapānaṃ guḷikābaddhaṃ.
And as each century passed someone would remove a single mustard seed.
Tato puriso vassasatassa vassasatassa accayena ekamekaṃ sāsapaṃ uddhareyya.
By this means the huge heap of mustard seeds would be used up before the eon comes to an end.
Khippataraṃ kho so, bhikkhu, mahāsāsaparāsi iminā upakkamena parikkhayaṃ pariyādānaṃ gaccheyya, na tveva kappo.
Assuming the span of a kappa can be can be calculated using above simile;
The equivalent length of a yojana as about 13 km (8 mi) throughout his translations of the Bhagavata Purana.
And “Karma vipāka” a book by Nauyane Ariyadhamma Thero says it is 16 mi.
Assuming it is 8 mi, the calculation.
However, according to Pabbatasutta the calculated value must be far greater than this value since a fine cloth from Kāsī would not be able to remove same amount to a mustard seed from the rock. Therefore, calculating a value using the simile is not that accurate.
This just is to have an approximation.
1 eon > 2.13410909 × 10^23 years (~213 sextillion)
Ajahn Brahmali’s calculation shows it is 36.8 billion years. 213 sextillion >>> 36.8 billion