Er, that's kind of the problem that Stalker0 was talking about. A DM wants a climatic battle with a single opponent so it needs to be tough enough to survive 5 creatures attacking it at once, yet previously toughness tracked with power and thus, said creature if it actually got to go first, would obliterate the party.
Yep, I agree with him. Definitely mixed results on the solo. Making them tougher was a good goal. I think in some cases they have been made too tough without enough oomph to match, so it's like the party is just sitting there, plinking arrows at it (or even "Lightning Bolt! Lightning Bolt! Lightning Bolt!") until the big lug falls. There's not much tension because the BBEG doesn't really have a lot of firepower, just a lot of toughness.
That said, I'm curious about something...
What do you consider "too long" for combat against the BBEG?
In terms of number of rounds and in actual real-life time, how long should the climatic battle be?
Well, like a lot of people, my first thought would be that it's "too long" if the players or the DM get bored. Of course, there are a lot of things that a good group can do to keep combat interesting, everything from great narration and descriptions to dynamic terrain to monologues to cut scenes. As a general rule of thumb, I don't want the climactic scene to take more than, say, 20 or 25% of the session in real time. So an hour, hour-and-a-half would be my rough idea.
But that's very rough. I've DM'd and played in 4-hour combats and enjoyed them. I've also been bored silly by 30-minute encounters. I think a few things added to monster design for solos would help:
1. More threat ability. This doesn't have to mean hit points and death. It could mean interesting curses or conditions that last outside of combat, like you've been partially banished to another plane so until the curse is removed you have a 1 in 6 chance of not acting in a round, standing mute when you're trying Diplomacy or something else in a skill check, or something similar.
2. Explicitly state their terrain-altering abilities and describe the effects. People have been using flavor text to collapse roofs of caverns for almost forty years. Instead of explicitly stating that the flavor text has no combat effect, how about explicitily stating what those (really cool) combat effects are? I'd love to see a dragon that could collapse a cavern on the adventurers by sweeping its tail through the supporting stalactite-stalagmite columns.
3. Minion rules for players. I'm serious. I'm not sure how... but the idea intrigues me.

("My henchman... no!")