It usually takes around 12 years minimum. You can check visa bulletin which can give you prediction. If its says 12 years, that means if you applied 12 years ago your sibling can get the visa now. But if you apply now it might take more than 12 years. They only allow 65,000 visas per year and 7% per country. Its a long wait.

How long does it take to get GC for parents who are currently living in Nepal? Any Idea?

I believe the latest timeline is 2006 as per the USCIS website, but YMMV for Nepalese....

Parents comes under immediate relative, so they don't have to wait for visa to be available. All you have to get is USCIS approval which takes around 8 months for which you have to file i130 for each parent and NVC approval. Its a matter of process, usually takes less than 14 months.