Well, one season for a theatre usually equals one year. If a play goes well, it’ll keep going as long as there’s an audience - the No Man’s Land production with Ian McKellen and Patrick Stewart premiered in 2013 and is still going, and there was a Cologne production of a one-man play that went on for decades. It’s quite possible that one season/one year for one play in the normal run of things equals only as many or even fewer performances than Hiddleston does in his three weeks of Hamlet, if they have performances almost every night over those three weeks.
Of course, when you’re doing theatre with a school’s drama group, you often do only one or two performances. But then again, those kids are not international movie stars like Hiddleston is.
And the thing here is, the role of Hamlet has many, many lines indeed (even if you cut down the text by half or something).
EDIT: To give a rough impression, Hamlet has double the lines that Macbeth does: 1476 to 690. It’s a lot to learn by heart, and Hiddleston has never played the role before. So basically, Hiddleston must really have fun at learning Shakespeare lines.
If you want to gain an impression of how much work this is, try learning just the famous soliloquy by heart and see how long it takes you until you have it:
HAMLET: To be, or not to be–that is the question:
Whether ‘tis nobler in the mind to suffer
The slings and arrows of outrageous fortune
Or to take arms against a sea of troubles
And by opposing end them. To die, to sleep–
No more–and by a sleep to say we end
The heartache, and the thousand natural shocks
That flesh is heir to. ‘Tis a consummation
Devoutly to be wished. To die, to sleep–
To sleep–perchance to dream: ay, there’s the rub,
For in that sleep of death what dreams may come
When we have shuffled off this mortal coil,
Must give us pause. There’s the respect
That makes calamity of so long life.
For who would bear the whips and scorns of time,
Th’ oppressor’s wrong, the proud man’s contumely
The pangs of despised love, the law’s delay,
The insolence of office, and the spurns
That patient merit of th’ unworthy takes,
When he himself might his quietus make
With a bare bodkin? Who would fardels bear,
To grunt and sweat under a weary life,
But that the dread of something after death,
The undiscovered country, from whose bourn
No traveller returns, puzzles the will,
And makes us rather bear those ills we have
Than fly to others that we know not of?
Thus conscience does make cowards of us all,
And thus the native hue of resolution
Is sicklied o’er with the pale cast of thought,
And enterprise of great pitch and moment
With this regard their currents turn awry
And lose the name of action. – Soft you now,
The fair Ophelia! – Nymph, in thy orisons
Be all my sins remembered.
[Obviously, learning stuff like this by heart is part of an actor’s training as well as natural skillset more than for any other person, but still. Just try it.]