It all depends on the group and the campaign. For my big 3.5E campaign, it was a new group, so I didn't plan anything out too long term, but had a few general ideas of where things could go if it developed into a campaign. I started off with a low level adventure, and as the group went through the adventure, one guy playing an elf PC decided he wanted to become a Paladin of Freedom down the road (to oppose the evil theocracy nearby that was responsible for much of the world's slave trade) and a guy new to the group had a background as an escaped slave... so, it naturally developed into opposing the evil theocracy of slavery.
Not long after that intro module, I had an encounter with the slavers and some low level priests, and then also gave them a drow duskblade as an "unusual" type of muscle for the slavers. (I had the drow Champion of Elistraee mini, so I had to use it, of course@) The combat was a big one, and the slavers fell one-by-one until only the duskblade was left - but she proceeded to then drop the paladin of freedom (below 0, but not dead) and seriously wound another PC. She was then cornered by the remainder of the party, but cast her Swift Expiditious Retreat spell and escaped their grasp. Rather than pursue, the players tended their fallen comrades and some injured slaves.
The slavers struck back a few levels later (the PCs went into town to deal with a threat there, and came out level 4 or 5 instead of 3). The slavers were some higher level priests, some soldiers, as well as some orc scouting parties, a hill giant and (of course), the drow duskblade looking for revenge. She again dropped the elf paladin of freedom below 0 (hitting -10, with his death number being -11), as well as a key NPC ally before the players managed to drop her to exactly 0 hit points. It was a huge combat on my 4x8 table, so the drow was in one corner of the table with the fallen elf and NPC. I think the other PC in the area got dropped by another bad guy (2 PCs died and the elf almost did.) But, when the drow was at 0, the players were finishing the combat on the other end of the board, while one of the other fleeing bad guys helped the drow to escape. Of course, by this time, the guy playing the elf paladin of freedom really hated the drow. (not to mention the guy whose PC got killed by her...)
Cut to a few levels later, I leveled the drow up so she'd still be a challenge to the players, and the party encountered more slavers. I forgot the exact situation, but this was their first real encounter with all smart bad guys (no orcs, hill giants, goblins, etc). In the past, the orcs or whoever would just rush at the front line fighters while the evil clerics and the party casters would exchange spells. This time, the drow was invisible and appeared behind the party cleric (you should have seen the look on the face of the woman playing the party cleric as I dropped that Champion of Elistraee miniature behind her cleric's mini... priceless. Made the entire encounter prep worth it to me as a DM.) But, the drow did a full power attack and arcane channeled on her blade as well and did something like 58 points of damage - the cleric had 53 hit points. It was the first time the party healer had gone down, so the group was in a frenzy trying to save/heal her and also take care of the drow and the wyvern mounted cleric lobbing spells at them. Fun encounter, but when the cleric's troops all were killed, and he and the drow were the 3.5E of bloodied, they flew away on the wyvern.
So, by this time, they all really, really hated that drow duskblade. And, each escape was not cheesed up at all. It all worked within the context of the game.
They had two more encounters with her, and I forgot how she escaped that first time, but the final showdown was when everybody was level 17 and the drow was part of a drow raiding party that I made up to be the drow equivalent of the PC's party, though with a few twists, and a few hill giant barbarians in the mix (the PCs had some NPC allies). It was a fitting final showdown, with the drow getting dropped and then revivified and healed before finally getting killed in the end. I think the final blow was even struck by the elf paladin.
However, the drow was not the BBEG - she was just one of his major allies/minions, and a focal point of the players. The PCs knew early on that they were not ready for the BBEG (after that initial adventure, the PCs found out they were heroes out of prophecy, but were also warned that the BBEG had a pit fiend for a bodyguard, which they found out was way beyond them at that time.)