As Michael has explained, you must write the current page before crossing a page boundary.

So you design your function for ease of calling:

int16_t eeprom_writebuf(uint8_t slave, uint8_t *buf, uint16_t n);

and let the function handle the boundaries. You either have a Compile-Time constant for the page size, or you pass an extra argument to the function.

As you can see from my example timings, your worst case is only the extra page-writes for straddled boundaries. 256 bytes with maximum time of 6 + 5 + 5 ms.

David.