This curse was actually cast by Nyx, half in jest, half in curiosity, and it sapped the life out of everyone in her village, siphoning their remaining life into them. She essentially will live for the duration that all of her village would live for, combined, and will not grow or age in this time, living out literal life sentences. I feel that substituting sleep with caffeine is a lot like drinking unicorn blood to stay immortal. You will have but a half life, a cursed life, from the moment it touches your lips.

• Half A Life Summary

'Half-life' is a term taken from nuclear physics. When radioactive isotopes decay (become isotopes of other elements) they do so exponentially. The half-life is the time it takes for half of the original mass of the isotope to decay. The quotation in the OP shows that J K Rowling does not know the difference between a 'half life' and a 'half-life'.

Half A Life Summary

To be consistent she should also have written the equally wrong 'cursed-life'.In your sentence it is probably intended to mean that in 'your profession' half of the new entrants to the profession will have dropped out within two years. Of the remainder, half will drop out within two years. Of that remainder half will drop out within two years.