A guinea pig should be at least 3-4 months of age before they are allowed to mate. Males and females get along quite well, so it is best to leave them alone until the female becomes pregnant. It will take 9-10 weeks to produce the babies. Provide plenty of water because the female will drink more than normal.
The first thing you should know is if there is a market in your area for guinea pigs, otherwise you're going to end up with a LOT of animals. You should draw up a plan listing ongoing expenses because you'll have those whether you have sales or not so this may be an additional drain on your budget.
Breeding guinea pigs is not all that difficult as all you need is a male and female in the same cage. When you buy a guinea pig from a pet store if the males and females are in the same cage the chances are that you have gotten a guinea pig that is pregnant already.
Don't do it. It is cruel and inhumane; please find another "second job". There is a 20% fatality rate with ALL pregnancies, even with prior successful ones; babies often die, and inbreeding can cause many problems. Find something more responsible and animal-friendly to do.
First off, don't breed more than you can sell/give away. Too many animals in this world are neglected or homeless. Guinea Pigs have a short gestation period. Remember to separate your female from the male so you don't breed her again too soon. Mothers will also breed with male offspring if they are breeding age.
Guineas carry a larger number of complications in breeding than other animals. Females are capable of breeding at 4 weeks old, but can die if they are 8 months old.
Guinea pigs can have litters of anywhere from 1 to 8, so you need to make sure that you have plenty of cage space. Also, you will have to seperate the male and female once she gives birth and you have to move the boy babies after three weeks.
You shouldn't breed because that can cause sever illnesses or possabley death. over 5 fiths of female guinea pigs die due to breeding. Also if you DO breed the female should be over 1 month and younger then 9 months.