how much is 1 purvkoti

If you mean 1 purva in Jain cosmology, it is traditionally taken as:

1 purva = 84,00,000 purvāṅga = 70,56,00,00,00,000 years That is about 70.56 trillion years.

If you meant a different Jain term by “purvkoti”, it is not a standard time unit in the usual Jain reckoning.