。
She stared at me for a long time.
Literal
She [topic-は] long-time me [object-を] fixedly looked-at.
長いこと is a casual variant of 長い間 ('for a long time'). じっと is a 擬態語 for staying still and focused — here describing an unwavering gaze. じっと見る = 'to stare fixedly.' The combination of duration (長いこと) and intensity (じっと) makes the gaze feel weighty and uncomfortable.