I want to preface this answer by saying that I am not an expert, just a college student who is currently doing research for a historiographical essay on Norman castles. My answer will be specific to castles built in England from 1066 till the late 13th century. French, German, or Spanish castles may not follow the same rules as the English castles, though I would expect there to be a large overlap in construction time, methods, cost, etc.

I will do my best to address the question in three parts. Firstly, what kind of castle are you looking to build. Secondly, how are you measuring the time. Man hours is a good start, but Medieval construction was a seasonal activity, so the notion of "years to build" is difficult to use. Lastly, we need to decide what steps in the construction will be counted as the "man hours" required.

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Accounts from the 1066 Norman invasion of England claim that William the Conqueror constructed a castle in eight days, shortly after he landed in England. This is truly an impressive claim, though calling the fortification a castle may be misleading. It is unclear if the fortification built at Pevensey was a full Motte and Bailey, or just a ringwork. Compare this time to the construction of Dover castle, a huge stone fortress, over a period of twelve seasons from 1179-91, and you can see why the type of castle being constructed matters greatly. Castles that were "built" after 1080 were usually existing castle sites that were gradually improved upon as technology allowed or conditions required. Castles could also vary in their defences. The Motte of a Motte and Bailey castle could be built on a relatively small 5 meter tall scarp, a steep hill which was usually man made. Or it could be built on a massive 30 meter scarp, which would take far more time to construct. The Bailey of a castle could also vary greatly; both in the area covered and the depth of the ditch. If the keep or wall is made of stone similar design variation also exist. The thickness of stone keep walls could be as little as 2.5 meters or as great as 7 meters. Keeps could be a simple one story building with a basement and tower, or a four story structure with multiple towers, entry ways, and machicolations, defensive structures atop walls or towers that allowed defenders to safely drop projectiles on the attackers. The outer wall of stone castles had similar variations in area covered, height, thickness (curtain walls were generally thinner than the walls of the keep they surrounded), and complexity of defence.

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Moving on to time measurement, man hours is a reasonable method to use as mentioned above. However, the medieval work day was not set at 8 hours per day. The hours suitable for work would be affected by weather, lighting, and religious days. Generally, construction happened from spring to fall, when the weather was suitable and light was plenty. On a dry summer day, this could result in twelve or more hours of construction work on a castle. On a rainy day, construction may stop entirely. There is also the problem of motivation. Laymen in medieval times were often "drafted" into castle construction, so their willingness to work may greatly affect the number of hours needed to complete a castle. The castle Chateau Gaillard, constructed by Richard the Lionheart from 1196-98 in one and a half construction season, was a massive stone castle meant to protect Rouen and the surrounding Norman lands from the French king. Richard is believed to have personally overseen the construction, constantly urging the workers to labor faster, greatly increasing their efficiency.

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This last point is a little nit picky, but it is still important to understand. Construction materials had to come from somewhere, and while not immediately involved in the construction of the castle, quarrying and transporting stone was a laborious process that if included would massively effect the man hours required to build a castle. Much of the stone in English castles was not mined near the construction site, with some castles even being built from stone imported from Normandy. This man hour cost also applies to wood used in palisades and floors, iron needed to plate the portcullis, and time needed to craft the missiles that were essential for any castle's defence. Unfortunately, I am unfamiliar with the average time needed to produce one cubic meter of stone fit for castle construction, or any of the other essential materials that all went into castle construction, but were not done by workers "on site."

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This provides us with the very unsatisfactory answer of it depends. Castle meant different things in different periods of time, so the man hours required varied accordingly. I will provide you with the best numbers I have come across in my research, but take each with the largest grain of salt in history. If we are looking at ringworks, 72,000 man hours is a rough estimate. Early Motte and Bailey castles could require anywhere between 12,000-288,000 man hours. I have not come across an estimate for the man hours needed to construct a stone castle, but they are undoubtedly higher than the time needed to build a Motte and Bailey.

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Resources used to provide answer.

Brown, R. Allen. 1956. English Medieval Castles. New York: Simmons-Boardman Publishing Corporation

English, Barbara. 1995. "Towns, Mottes, and Ring-works of the Conquest." THe Medieval Militar: 45-61

Stockstad, Marilyn. 2005. Medieval Castles. Westport, Connecticut: Greenwood Press

Taylor, A.J.. 1969. "The Norman Conquest and the genesis of English Castles." Château Gaillard, 3: European Castle Studies: Conference at Battle Abbey: 1-14