Going to Namibia for 3 months after being in South Africa on a 90 day visa

Question

I want to know if the neighbouring SA countries I.e Namibia treats the 90 day Visa waiver programme the same way South Africa treats it ? For example I have a U.S passport..I entered South Africa via Johannesburg and intend to stay almost for 3 months then plan to go to Namibia also for 3 months on the visa free programme. Will I get 90 days on arrival in Namibia even though I spent almost 90 days in South Africa ? Or would Namibia customs factor in how many days out of the 90 I have already spent in South Africa before determining how many days i can stay in Namibia ? Any responses will be appreciated..thanks