It's take about a million years, and it's mostly because of solar wind.
The Earth has a Magnetic field, thats why we're save.
Even venus is constantly trailin large amounts of Atmosphere, but with the current pressure, that'll take a while.
Regarding biosphere, if you assume that, you also have to assume that there is a bipsphere to begin with on any planet where you
can actually build a new one, and that poses new problebs.
What is the entire planet is covered in a furry fungus producing large amounts of toxic waste every day?
You'd have to blanket the planet with nukes to plant anything.
On a barren rock with just enough gravity to sustain an artificial Atmosphere, in turn, it might be impossible to install a biosphere without importing way more than just gas.
And hey, while we're on it, a bigger planet should need way more gas to create an atmosphere.^^