Is there any indication of a grade or power level she has achieved? Perhaps in a letter from Princess Celestia?
The only thing offhand I can think of that would fit this description is her status as Princess Celestia's apprentice/most loyal student (see the character sidebar and summary on Twilight Sparkle's wikia page).
Twilight Sparkle is a unicorn pony who is a special and loyal apprentice of Princess Celestia [...]
Is her level of magical aptitude normal for a unicorn?
She's actually quite capable with magic in contrast to most unicorn ponies, who usually only get a basic couple of powers (mainly telekinesis AFAICT) and one or more special spells related to their cutie mark (for example, Rarity's gem finding spell).
She has shown that there are limits to the power she can exert (only barely being able to levitate the Ursa Minor back to its cave in Boast Busters being a prime example), and she's demonstrated that she can quickly learn even complex spells (glancing at the time travel spell scroll in It's About Time, then nearly immediately casting it).
As far as present limits, it seems to vary based on the spell in question. A few examples:
- Twilight's teleport spell is dependent upon distance traveled and what she's teleporting with her. In Dragon Quest, she exerts considerable effort to teleport her, Spike, Rainbow Dash, and Rarity to safety (this is as far as Twilight has attempted to stretch her teleport spell), whereas she can ordinarily use her teleport spell with next to no visible effort for short-range teleportation. (An aside, if she's not careful with how she uses it, the teleport spell will drag others nearby with her, with somewhat unpleasant results)
- She apparently has the ability to make cutie marks appear on ponies who don't have them, with effort expended roughly corresponding to speed at which they appear. (this spell is of questionable utility, but shrug)
- The effort required to use telekinesis seems to vary based on the weight of the objects being manipulated. She is seen in Applebuck season harvesting an entire orchard with little effort, while the Ursa Minor was difficult to move. (entirely speculation would be that it takes more effort to lift large objects, and more coordination to manipulate things in quantity)
- She also had the ability to manufacture facial hair (Spell 25), and if memory serves she progressed fairly quickly with the use of that spell throughout Boast Busters.
- She gained the ability to fire what are essentially lasers from her horn as of A Canterlot Wedding - Part 2, ostensibly through off-screen/inter-episode studies. (side note: this results in the most awesome clip ever)
As a more general statement, I would speculate that Twilight is running into spell limits before she's running out of energy to execute them (given that she easily cast the aforementioned time-travel spell, and the only thing stopping her from reusing it is the spell's once-per-caster limit). Just conjecture though, since the science of unicorn magic isn't well defined to us viewers.
Twilight is exceptional in regard to magic, having dedicated herself to studying the art. Spike mentions in Boast Busters that Twilight's magical ability is exemplary, and Princess Celestia comments about Twilight's enormous raw magical power in The Cutie Mark Chronicles. Unicorn magic requires concentration and effort, as Twilight explains to Spike at the beginning of Feeling Pinkie Keen and to the Cutie Mark Crusaders in Call of the Cutie, [...]
Twilight is shown developing her skills in a flashback in The Cutie Mark Chronicles, where she has some difficulty using magic to turn a page in her book; the scene shows her progressing in her studies, a theme which continues throughout the series.
Twilight possesses the ability to teleport, which was also common among previous-generation My Little Pony unicorn characters. She first teleports on the show in Friendship is Magic, part 2 when she charges Night Mare Moon and disappears only to reappear behind her and next to the Elements of Harmony. She teleports short distances in Applebuck Season to get ahead of Applejack trying to confront her about her over-exertion, and she unintentionally teleports with Spike from the thoroughfare into her library in The Ticket Master, which leaves Spike slightly singed. She teleports multiple times in Lesson Zero, seemingly without effort. As seen in Secret of My Excess, she is able to teleport other beings as well, teleporting Spike several times to clean a lipstick mark off his cheek.
[quotes sourced from the My Little Pony wikia]