I definately prefer a faster leveling rate at the lower levels. The PC's are too easy to kill. It's difficult to make enounters challenging without making them lethal when one well-placed axe or crit will kill a poor fighter with 10-20 hit points. After a few encounters, it's easy to understand what your characters can do since there are few options to really choose from. Once every 10 hours of play is pretty fair, IMO.
Once you get to about level 5, it can slow down a bit. The characters have more options, they're a little tougher, and you can throw a greater variety of problems at them. Maybe every 15-20 hours of play.
For my money, levels 8-12 are the golden era of a game's lifespan. The characters can do some amazing things, but nothing too off-the-wall or potentially unbalancing. You still have a general idea as a DM of what they're capable of and can have it mind when preparing for a session. I would expect or hope to level once every 20-25 hours of gaming. While level loss may occur at this stage of a game due to the characters now having the resources to raise a comrade from the dead, I consider it a fair price to pay at 20'ish hours of gameplay (unless it's a particularly challenging campaign and the PC's are having a little trouble staying alive). Any longer than that, and it's simply too staggering a blow to think that I'll be sitting through another 40+ hours at a gaming table without any tangible progress to my character. Progress to the story, perhaps (which is all many people consider important -and I agree to a point), but call me crazy if I feel a little giddy when my character levels and I get to make him more powerful and utilize new, cooler abilities.
At levels 12-16, the PC's level of power is even more fantastic, and the DM walks a spider silk-thin line of making the game challenging but not killing the party with the degree of power opponents will have. Still, I would keep the progression of leveling the same for the reason I mention in the previous paragraph.
After level 17, it's no holds barred. Characters can True Res, they have absolutely amazing, game-wrecking powers that can get out of hand easily. It takes a brilliant DM to be able handle this level of power and still make the game challenging to the PC's, both from a combat and an story standpoint. Killing a PC might happen a lot more often, but the consequences are relatively paltry. (Keep in mind, even if there's not a cleric the party, they have the money to pay for a True Res if they can find a cleric who can cast it). As far as the pace of leveling, typically your campaign is winding to a close at this point (the ELH may change this) so it may or may not matter. Level advancement is entirely to taste at this stage. Are the PC's going to turn into NPC's after this -who may cameo in later campaigns? Retire and fade into obscurity? Die in a blaze of glory (and stay dead this time)? Whatever the plans, I would say that if the game truly is winding to a close, why not be a little more generous? If it's not, stay with the 20-30 hours rule-of-thumb, or maybe up it to 30-40, whichever. Either way, it's not going to matter much at this point as the character's abilities are nearly as good as they're going to get.
That's my ten cents.
