Hello Michael,
This section of the datasheet is intended to ensure that the device is properly initialized, both analog and digital portions. Upon manufacture, One-Time Programmable (OTP) registers are set internally to program characteristics such as the device ID and trim settings for an accurate internal oscillator and reference voltage. Failure to initialize the device correctly may cause an ADS1298 to think it's an ADS1294 (ignoring the other four channels), or /DRDY to pulse at the wrong frequency, just to give an example.
That said, it's always safest to wait slightly longer than risk resetting the device too soon upon power up. However, 820ms sounds awfully long to me. If you see Figure 53, the value of R1 (internal) is either 12.5k or 10.5k, depending on the reference voltage. For the recommended 22uF capacitor on VCAP1, this yields a max RC time constant of 275ms. What size capacitance are you using (I've estimated 70uF - 83uF)? Larger capacitance will give you the benefit of better bandgap voltage filtering but at the cost of longer tPOR time.
