[vxNsr]

Alternatively, the OP means he spends 10% of his work time on this, not 10% of the 8760 hours in a year on it.

[kqr]

10 % of their work time is not a very well defined amount of time, though. If this is their only occupation, then it's 100 % of their work time. If they have a different primary occupation, then 10 % of that depends entirely on how much they spend on that!

[vxNsr]

You might be over thinking this, most people think of “work time” as 40 hours a week. So just think 10% of that, i.e. 4 hours.