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the thing to remember is, that USB on a computer only provides 500mA PER ROOT HUB, unless you are using an externally powered USB hub.
so if your laptop has 4 USB ports, most likely, there are 2 ROOT HUBs on the motherboard. If you plug your cable into one, and plug, say, a USB mouse into the port right next to it, some of that 500mA is powering that mouse, giving the charging cable less than 500mA. Plugging into the other set of ports will probably charge faster.
note: manufacturers differ, mileage may vary, stuff happens </disclaimer>.
Cable impedance is also going to limit the power available on the business end - all cables have some degree of impedance. Thats is why most usb charging cables are only 3-4 feet long.
my wife's kobo takes about 4-5 hours to charge from the "plug me in" screen on my desktop. sometimes longer, if power saving kicks in. From a 800mA call phone wall charger, it takes about 2.5 hours. the pc will lower the voltage if you are not actively using the pc - especially if you are on a newer OS, like Win7 or OSx (insert cat name here).
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