Well, it depends on the limits set by Marketo for the API calls: for the regular non-bulk APIs, you can query 300 records for the lead DB endpoints (the query CO endpoint falls within this) and 200 records for the asset endpoints in a single call.
However, for the bulk extract endpoints (CO bulk extract endpoints), the default daily quota is a maximum of 500MB per day (you have the option to extend this limit against a charge if need be), shared between leads, activities, program members, and custom objects. When the quota is exceeded, you cannot create or enqueue another job until the daily quota resets at midnight Central Time. Until then, an error “1029, Export daily quota exceeded” is returned. Aside from the daily quota, there is no maximum file size.
You can check how much Bulk Extract API capacity has been taken up by your Marketo Engage instance over the last seven days by going to Marketo Admin > Web Services > Bulk Export API information. Here's the link to the product doc that illustrates the same.
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