Based on how long it took for the Earth to generate an atmosphere with free oxygen I think you are looking at centuries. And that is with the maximum amount of genetic engineering to do what you wish. It took over a billion years for the Earth to generate it.With genetic engineering and exponential growth it could be done in a short time.
Imagine genetically engineered bacteria that take in CO2 and release Oxygen and Nitrogen and thrive in the Martian habitat?
The planet would be covered in Oxygen in a couple of decades tops, lol.
You are not really thinking about the exponential growth thingy. The genetically engineered bacteria would have ZERO PREDATORS.
Other than the incredibly hostile environment, and the fact that Mars is pretty darned big.