When the race will end

May 20.

By pledged delegates, Clinton is around 159 delegates down. Giving her the most favorable difference ratings in the upcoming states (and splitting Guam 2-2, not that it matters), Obama will have an insurmountable (i.e., more than all the remaining delegates) lead after the Oregon/Kentucky primaries on May 20 -- I'm giving her a pickup of 18 in Pennyslvania, 6 in Indiana, loss of 9 in North Carolina, pickup of 8 in West Virginia, 15 in Kentucky, and 4 in Oregon. These are all from the most favorable for her recent opinion polls in those states that I could find.

These numbers are pretty stable. It will be almost impossible for Obama to have an insurmountable lead before May 20, because there are 189 delegates assigned on or after May 20, and his lead would have to grow by 30 delegates before then, which is impossible unless Clinton simply collapses. It will also be almost impossible for Obama not to have an insurmountable lead after May 20, because after May 20 there are only 86 delegates left to be assigned, and his lead of ~160 isn't going to be halved with an assignment of 480 delegates -- that would require Clinton win two-thirds of the delegates.

All these are subject to Obama not being hit by lightning or being found out as a secret Buddhist, of course. But barring that, Obama's pledged delegate lead will be impossible to beat on May 20. Then there are 11 days before Puerto Rico votes. Since Puerto Rico isn't a state, and Montana and South Dakota are red states, and so don't "count," Clinton's camp should be ok with the superdelegates making their decision on May 21.