You don't choose, the ASSYST decision is based on a number of factors and that in turn dictates whether the count-down is displayed in miles or days.
To put it simply if the car has covered very few miles and the maximum time between services is approaching, 24 months I believe, then the ASSYST displays a count-down measured in days. Whereas if the car has covered more miles approaching 10-12k miles I think, or the oil is showing signs of degradation then the ASSYST will display the count-down to the next service in miles.
S.