Each time he shaves his head, he gets balder. Hope this helps! The very first time he does it is very important, it’s about him accepting his new identity when it comes to his cancer and his.
When Does Walt Shave His Head
Whatever drug he was on in the earlier seasons did make his hair start to fall out prompting him to shave his head. And even then he still had eyebrows and facial hair so it wasn't particularly quick.