Hours -- most of that day, actually. You will have lines to wait in, get your ID made, try on uniforms, fill out paperwork... No single thing really takes a long time (minus the waiting in line if it is really busy that day), but you have a lot of different things to do that day. If you get there at 8:30am or so, you ought to be done -- including wardrobe and checking into housing, if not unpacked -- by about 3pm. Your office, depending on your department, may want you to watch training videos and such, so that might take a bit more time. In no way will it take you longer than until about 5-6pm if you start between 8-9:30am though.
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PO!NT OF VIEW - A Different Look at Roller Coasters
http://www.crosswinds.net/~justmayntz/thrills/