。
She had been taking piano lessons for a long time.
Literal
She [topic-は] long-time piano [object-を] was-learning.
習う ('to learn / take lessons') implies learning from a teacher, not self-study — distinct from 学ぶ (which can be self-directed). ピアノを習う = 'to take piano lessons.' The polite past progressive ~ていました suggests this was an ongoing activity that has since ended — she's no longer taking lessons.