How often should you give worm medication to a 4 to 5 lb toy rat terrier that is 6 months old? ::)

As often as your vet says he has worms.

Please do not regularly worm any puppy that doesn't have worms. Wormers are poison, and with such a small puppy you are moving in dangerous territory.

Having a pup tested for worms is inexpensive and happens while you wait.

Puppies should be wormed when they have signs of worms. In addition to that, unless you live someplace blessedly free of mosquitoes, it should be checked for heartworm as soon as they appear. Then put it on a preventative. I keep my puppieson Heartguard + year around. That way I do not have to have them checked for heartworm and the Heartgard+ has ingredients that protect against other worms too.