How long until most breeders find out if the dog is pregnant?

The breeder I want to buy from bred her dog on the 20th and said she'd let me know when she found out if she was pregnant. I was trying to find out when it would be without pestering her but I can't get a straight answer on the internet. Also, how likely is the dog to be pregnant? She's a 7 lb pomeranian who's had one litter of four before the breeder bought her and she was bred to a 6.5 pound chihuahua that's been a dad before. I don't know if the size and type of the dogs affect the chances or how long I have to wait to find out?