6 years/500,000 kms according to my local Taxi depot manager and then they are essentially scrap or flogged cheaply .
Residual value has never been a major consideration in the Taxi Business because having had a taxi history buggers the car's fair market value regardless of KMs....in fact if a Motor Dealer sells an ex-cab she has to obtain a written acknowledgement from the buyer (let the buyer beware)
Apart from insurance, rego and Brand fees, the biggest expense by far is fuel followed by R&M. To date this is why the Toyota Hybrids are favoured. Some Taxis on weekends operate 2 x 12 hr shifts so for them battery charging is a problem. For example a Tesla takes about 8 Hrs to fully charge!
A Taxi plate alone costs up to $600/Week to lease!