A K-girl agency in my area has been on "vacation" since Christmas. Is it typical for an Agency to be on vacation for this time of length? Not that I don't think they deserve a vacation but just wondering. I will admit that I'm getting antsy to see what talent they bring next

About once every 3 years she'd go back to Korea for about 6 months.

I don't know if you mean an agency that makes appointments for their staff at numerous distributed satellite locations (apartments, hotels, motels, ...). Or do you mean MPs with a fixed geographic location and a rotating group of staff members?  

If the former, it might just be the booker / phone girl who is, in fact, on vacation and someone else (different number) is doing the scheduling for now.  

If the latter, I can only think of NYC. Once a while, a shop (often in commercial space) will close in order to do elective renovations: upgrade plumbing for a new washing machine or table shower room; upgrade electric to avoid a code violation ; new furniture; partitions, painting, etc.. If there was trouble in the building (a fire in the pizza shop below - actually happened; a burst frozen water pipe; almost anything), they have to close to fix things up.  

If you are referring to an ad or phone number for a specific agency WITHOUT visiting their geographic location, it might be that they lost their lease or got busted. Now, they are on "vacation" scrambling for a new apartment or loft which likely needs to be upgraded to KMP quality (plumbing, table shower, etc.). So far, you're talking about 2-3 weeks; a not unreasonable time to get building permits and to get the job done.

When you reconnect to the same number, you might find yourself being told to go to a new location.

K professionals have clients jump through loops while on vacations.

An agency here seems to be virtual. They appear when a girl is available to tour through the the area. Then they disappear only to reappear again several months later. Since the girl's review dates tend to continue on across the country, it seems like the agency is virtual and geographically defined.