Guinea pigs can start mating at a very young age! We usually recommend breeding your pig BEFORE 6-7 months of age as their pubic symphosis or pelvic canal fuses shut after this, and this makes it much more likely that your pig may have a dystocia (difficult delivery) resulting in a CSxn!!
Pigs can start mating as young as 1-2 months of age, and typically go into heat every 2 to 2 1/2 weeks. They are pregnant on average for 63 days. Normal litter size is 1-2. Bigger litters are not ideal, as pigs only have 2 teats for piglets to nurse on!!
Your pig should be on an excellent nutritional program and daily VITAMIN C supplementation should you decide to breed.
Most pigs will breed just shortly after birth, so be sure to remove the daddy pig when mommy pig gives a birth to their litter!!
Most guinea pigs can successfully breed for almost 3-4 years before fertility significantly begins to decline. I don't recommend back to back breedings, as I think it is stressful on the pig, and I think they need time to recover from lactation and the stress of parturition. Breeders will always have their own philosophy on this....vets will tend to sway in favor of what is best for the pet in question vs. the need to make pigs for sale....
Sometimes xrays of your pigs abdomen may be recommended to do a fetal count in case there are any worries of a dystocia or difficult birth.
Hope this information was helpful. Best wishes,