Giant γ-ray flares comprise the most extreme radiation eventsobserved from magnetars. Developing on (sub)millisecond time-scales andgenerating vast amounts of energy within a fraction of a second, theinitial phase of these extraordinary bursts presents a significantchallenge for candidate trigger mechanisms. Here we assess andcritically analyse the linear growth of the relativistic tearinginstability in a globally twisted magnetosphere as the trigger mechanismfor giant γ-ray flares. Our main constraints are given by theobserved emission time-scales, the energy output of the giant flarespike, and inferred dipolar magnetic field strengths. We find that theminimum growth time of the linear mode is comparable to the e-foldingrise time, i.e. ˜10-1 ms. With this result, weconstrain basic geometric parameters of the current sheet. We alsodiscuss the validity of the presumption that the e-folding emissiontime-scale may be equated with the growth time of a magnetohydrodynamicinstability.