At about 4 months old, ^ as mentioned. Don't use any soap or shampoo unless it is made specifically for guineapigs/small animals. Also don't bathe too frequently, it dries and irritates their skin. Once every 2 months or as needed. Hope this helped!
You should give your guinea pig a bath when she is about 4 months old. However, it is advisable to consult your vet about the appropriate animal shampoo that you should use in cleaning her in order to avoid any infection.