I figure Durkon can sustain at least 8-9 hits from Roy's sword, largely because he killed the clerics before the fight to buff his temporary hit points, and can sustain an additional hit every 5 rounds he drags it our and every 2 times he drains Roy. Durkon appears to have taken 6 hits, and dragged the fight out to about 8 rounds and drained Roy once. So he can still take 4-5 hits at this point.
Roy may have only 170-180 hit points. He's taken about 75 damage from harm, 30 from inflict serious wounds, 26 from flame strike, and 13 from a vampire slam attack. That's 145 damage. He's only in this fight at all because he's made every single saving throw he's been asked to make. Roy needs to hit everything on a full attack action against what is apparently Durkon's very impressive AC, or he's going down soon. From Roy's perspective EVERYTHING has gone wrong to this point. His only hope was to get in several rounds where he hit Durkon 3-4 times in the same round and keep Durkon within 5' of him at all times and disrupt multiple attempts at spell-casting. That hasn't happened, and he's now basically lost.