Air is fairly diffuse, as in there aren't nearly as many atoms per unit volume as with a liquid, so the atom will stay suspended until it encounters another element or compound with which it can react chemically and has an interaction with that other element/compound of a long enough duration that the interaction can proceed. Smacking into an O2 molecule "might" result in the formation of PbO but not if the collision is too forceful, too weak, or not adequately direct. Eventually, over time, thermodynamics say that the metallic lead in air will undergo an oxidation-reduction reaction and make some sort of base salt such as lead carbonate or lead oxide. However, it does take some time for that to happen on a statistical basis (never goes to total completion, really).

Even then, though, the new compound will tend to stay suspended until it absorbs onto some larger solid or dissolves into some larger liquid mass (like droplets in a cloud). It will then go wherever that larger mass ends up going.

If air is still (no movement at all), there will be a slow downward migration because of density differences, but even the slightest air movement will be enough to keep the atoms or tinier particulates in suspension.

Mostly though, the atom will bounce around in the gas, as part of the gas phase, until it gets lucky and reacts with some other participant in the chaotic dance, making a new molecule.

Even metallic lead has a vapor pressure, the presence of some atoms that will leave the solid and enter the air just by random energy pulses, so the drive to "rain" out of the air just by density isn't all that powerful. That is, in a closed space, if you leave a bar of lead out on a tabletop, some of that lead will escape the solid and enter the air. Not a lot because lead isn't all that volatile, but you cannot prevent all loss at the interface (surface of contact between air and solid).

I don't know what the average residence time (median duration that the atom would exist in suspension before descending to the ground) of a lead atom in air would be. Not likely a value measured in seconds or minutes. Even household dust has mean residence times longer than that, and that stuff is destined to fall fairly rapidly by comparison to a lone atom, if only the air would stay still long enough.