You must consider his experience and skill level. You could contact him and ask, but I would estimate, based on his time lapse, that he spent over 30 total hours start to finish with the sculpting, Polypainting, baking, and Photoshop tweaking. If you have not attained his level of experience and skill you could spend twice that many hours and still not have a character bust at that level of quality. If you start considering a full-bodied character at that level of finish with clothing/armor the time required would expand (2-3 weeks or more with a serious daily time commitment).
You cannot really judge how much time it would take you to complete such a bust without considering your current skill level vis-à-vis his. Also, if you are making a character for a portfolio and are not under a game character development deadline then the amount of time will also vary accordingly. That bust was also made about three years ago. He notes on a more recent full-body character that he used the Quixel suite and Mari as well. The "Priestess" model (posted a year ago) employed Substance Painter as well as Mari. You must consider his software knowledge as well as his artistic experience and understanding of anatomy and more. If you want to make game art at his level then you have a long journey ahead of you.
Using the referenced bust as an example, the sculpting in ZBrush would take at least 18-20 hours assuming you have a lot of experience with ZBrush and knowledge of anatomy. The fibermesh beard stubble is another consideration. A "sculpt only" is really not going to gain you much if you are doing game-related art (other than practice). If you mean "realistic" as exceeding expectations for a high-quality game character that would get you a job as an entry-level character artist then you will have to seek out advice from someone in the film or realistic sculpt-making fields.
Giving timeframes is a bit pointless when you don't know what it is about (quality level; experience; details - in your example the hood and other things;...). But also keep in mind - the quality level you need these days to get even a step into an AAA-studio means that you probably will have spent years learning it. In other words quality is the stepping stone, not speed. Speed will be there through sheer practice to reach the quality level (same goes for technical skills like UVing & retopo). I would say a regular AAA artist can get a realistic head sculpt within a week's work (without revisions or other things coming in).
Only the sculpting of the head starting from a basemesh takes me around 3-4 hours for an average quality head, like what you'd expect from an NPC that can sometimes gets some camera close up. If it's a more important character it can take up to 6-8 hours. A main character can take several days of sculpting if you include all the feedback.
Hope these numbers give you a better idea of how long it can take to make a realistic head, but you shouldn't worry about speed, quality will always come first.
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You cannot really judge how much time it would take you to complete such a bust without considering your current skill level vis-à-vis his. Also, if you are making a character for a portfolio and are not under a game character development deadline then the amount of time will also vary accordingly. That bust was also made about three years ago. He notes on a more recent full-body character that he used the Quixel suite and Mari as well. The "Priestess" model (posted a year ago) employed Substance Painter as well as Mari. You must consider his software knowledge as well as his artistic experience and understanding of anatomy and more. If you want to make game art at his level then you have a long journey ahead of you.
Hope these numbers give you a better idea of how long it can take to make a realistic head, but you shouldn't worry about speed, quality will always come first.