If hairs are lost because of some medications or drug abuse or any kind of supplementation then there will be chances of low levels of testosteron and other hormones is the cause(msinly hormonal causes). This can be 90% - 100% reversable with herbal extracts and more things. (workout,good diet etc.)
If baldness is due to normal ageing process then it will depend upon one's body,immunity and more to regrow…
In my case i lost my hairs some 4 years ago due to supplementation of something cheap type weight gainer and i regrown my hairs back. It took 4 years to regrow. 95% hairs are regrown still my scalp is having new hairs every month.
I used shilajit and other powerful herbs continue and now using dermaroller 0.5mm with onion juice as well both twice a week