You must update every element, so you are are looking at O(n) time regardless. On my system, updating 35,000 elements takes between 0.01 and 0.03 seconds, 350,000 items takes .1 to .3 seconds, etc. You can get a small performance boost by getting the value of DateTime.Now outside the loop like this:
var now = DateTime.Now;
foreach(DataRow dr in oDT.Rows)
{
dr["ModifiedOn"] = now;
}