It sounds great. Many schools offer credit transfers. Ask your college first. On the plus side, an agency can handle international payments - many accept credit cards, offers an international point of contact, and has checked several schools. On the downside, many of these agencies require your to prepay before starting a program and some charge a registration fee.
Courses can last from one day to several months.
Most schools encourage students to take at least one week of classes and preferably two or three weeks for maximum benefit. As always, it does depend on your abilities and needs.