Do you mean when the tiles are prefetched for the next scanline?
That's during cycle 320 through 335 (if you start counting from 0).
Search for "Memory fetch phase 161 thru 168" here:
http://nesdev.com/2C02%20technical%20reference.TXT
A "memory fetch phase" is two cycles, so fetch phase 161 means cycle 320 (brad taylor's doc starts counting from 1).