The only data I have found to start is the 2012 stats for Montgomery County—early voting in 2012:
Obozo: 62,233 80.0%
Mittens 14,403 18.5%
So that is one baseline.
Let’s see if we can find current early voting by party for comparison.
I just checked and Obama won Montgomery County 71-27%, with 287k-111k the final tally. So early voting was not a sizable factor in 2012. I fully expect Clinton to pick up roughly the same number of votes in Montgomery County. The only reason to suspect otherwise is that there is a *massive* hispanic presence in Montgomery County (I think it is a “sanctuary county”), and from what I have seen they may not vote in the huge numbers as they did for Obama.
What I really want to know is what the numbers look like in PG County, relative to 2012. In 2012, PG County was essentially a 1/2 million person extension of the Obama campaign circle. I cannot see her repeating his success there. The question is how much can Trump shave off from Romney’s deficit there.