If she gets married to a GC holder and apply for I-130,  I-130 processing time is around 5 months and is independant of her status. I-130 is not the GC approval. Then she needs to wait for around 3 years (per current visa bulletin) before she can apply for I-485. Till that time she needs to maintain her legal status in US (F-1 or H-1 or any other legal status).