Charm Person: This spell applies to all two-legged, generally mammalian figures near to or less than man-size, excluding all monsters in the "Undead" class but including Sprites, Pixies, Nixies, Kobolds, Goblins, Orcs, Hobgoblins and Gnolls. If the spell is successful it will cause the charmed entity to come completely under the influence of the Magic-User until such time as the "charm" is dispelled (Dispell Magic). Range: 12".The obvious features of both are: no duration given, and very long range. In fact, Charm Person appears to be nearly permanent, ending only when dispelled. I'd argue that that Sleep is open-ended; the spell ends under the same conditions that natural sleep would normally end (noise, physical contact, or getting enough rest.) Sort of like a hypothetical "Sit Down" spell, which causes targets to sit down; they remain seated until they decide to stand up. So really, Sleep is an instantaneous spell that causes a change of natural state, distinct from a possible higher-level spell called Enchanted Slumber which prevents waking (or the AD&D spell Suspended Animation, which slows biological functions as well as inducing sleep.)
Sleep: A Sleep spell affects from 2-16 1st level types (hit dice of up to 1+1), from 2-12 2nd level types (hit dice of up to 2+1), from 1-6, 3rd level types, and but 1 4th level type (up to 4+1 hit dice). The spell always affects up to the number of creatures determined by the dice. If more than the number rolled could be affected, determine which "sleep" by random selection. Range: 24"
I compare Charm Person to Sleep because they both affect living minds, so it's easy to see Charm Person as a change of natural state (friendship) as well. Crucial to this interpretation is that phrase "cause the charmed entity to come completely under the influence of the Magic-User". The common interpretation of Charm Person is some form of mind control, but I and several others stick to a strict interpretation of the word "influence". I run the spell as if it changes a person's reaction to the caster to Very Friendly and Loyal. The charmed victim then acts exactly like any NPC who rolled max reaction and max loyalty: no mindless obedience, no automatic friendship towards the caster's colleagues, and mistreatment or abandonment can trigger a loyalty re-roll. Thus, it's an open-ended natural state, similar to Sleep; friendship must be maintained or it will naturally decay.
The ranges break my nice little plan of 10 feet per spell level, but they are interesting; I don't think anyone's mention how far the long range of some spells are. The Sleep spell's range of 24" is 240 feet, which is just a few feet shy of the One-Page Dungeon standard format of 300x300 feet. That's an entire small level or sub-level! Charm Person has a range that's half that, but still significant, and I think a clue to the reasoning behind the range choices. Unencumbered humans, as well as many bipedal monsters, have a movement rate of 12", the same as the range for Charm Person; it reaches just far enough that a Magic-User has a chance to charm an opponent before it moves close enough for a mêlée attack. We might be able to keep the spell system simplified and regular by dropping specific ranges for Sleep and Charm and instead calling them (line of) sight spells, applying this open-ended range description to any attack spell.
A minor note on Charm Person: if we interpret "generally mammalian" pretty strictly, this means that the spell doesn't affect lizardmen and possibly not even mermen, depending on how you interpret them (more fish, or more man?) And consequently, the OD&D kobold is definitely not the little reptile guys of AD&D, since they are specifically named as charmable.
A minor note on Sleep is the sentence "The spell always affects up to the number of creatures determined by the dice." Some people interpret this to mean that if you cast Sleep on a small number of creatures, but roll high, members of your own party may be affected. It seems potentially amusing and may balance the perceived power level of the spell, but I haven't decided yet whether to go with this interpretation myself.
The final point to consider is the number affected by Sleep. Why so many? Charm Person affects only one target, and many higher-level spells affect fairly small numbers of creatures, with the exception of area spells. But Sleep affects an average of 9 HD worth of creatures (for the 1 HD and 3 HD ranges, at least.) I'm not sure yet if I will want to alter the spell to force it to match other spell effects, but I am thinking of simplifying the roll to 2d6 HD worth of opponents, with the restriction that only one creature can be affected if it is 4 HD.