What is the maximum speed I can write to the part?

The maximum speed at which a given device can be written to is given in the device's data sheet.

The AD9850 and AD9851 are spec'd in terms of clock and data setup-and-hold times.

The AD9852/54/58 parallel port can read data at up to approximately 100MHz (byte rate), while the serial port operates at up to 10Mbits/s.

The AD9951/52/53/54 and AD9859 have only a serial I/O port. This port, however, can be written to at a rate of 25Mbits/s.