Guinea pigs reach maturity around 15 months old. Once guinea pigs have finished the initial phase of quick growth and have got around 80% of thei adult weight and size, they stop growing fast and grow more in little spurts. This can happen any time between 4-8 months of age, depending on how quickly they did grow inially. Guinea pigs have the ability to grow until they are around two years old, but some very small piggies can stop growing from 6 months of age. Rex piggies typically grow for longer and later than other breeds. As the adult size and weight can differ enormously between ca. 800g to 1500g (on average about 900g-1300g), there are no hard and fast rules. Ideally, you keep your guinea pig on a nutritionally balanced diet to prevent them from becoming overweight (which can shorten their lives and pose an added risk in operations), but give them every chance to optimise their genetically determined individual potential.
As Wiebke says they vary quite a lot, my Jemima has only ever got to 850g (though that hasn't stopped her being top pig - maybe it's little pig syndrome). Bea was my biggest but naturally a long pig anyway, she reached 1100g before she got ill.
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