The main thing to keep in mind about charging any smart phone whether it be a Samsung Smart Watches, the iPhone Smart Watch or even the no brand name Smart Watch, the key item here is the mAh (milliamp hour). The higher the mAh the longer the charge lasts.
Some watches come with a physical USB charging cable and others come with charging contacts built into the actual watch. No actual cable is needed. You remove the one strap that covers the contacts and plug it into a USB port on the computer or to a USB port on a USB HUB. Most smart watches charge in about 2 hours. This varies with manufacturer. Also, some watches charge faster than others.
Below is a picture of the steps shown to charge a smart watch with contacts builtin. This eliminates the need to carry an extra item with you, the cable!