how much is 1purv koti

In Jain usage, pūrva-koṭi generally means a very large count of one crore. In some Jain numeration contexts, it is explained as a period of one crore years. ( wisdomlib.org)

So, in simple terms: 1 purva-koti = 1 crore = 10,000,000 in many Jain references. ( wisdomlib.org)

If you meant the time unit used in Jain cosmology, then it is used for an extremely vast span of years in the texts. ( jainworld.jainworld.com)