Could be similar to pharmacokinetics but I'm not familiar with the use of saturation there. You get this effect whenever you have an (almost) constant production and a decay that's proportional to the concentration. If we look at something like a month then Y-90 production is almost constant because Sr-90 decays slowly over decades. Y-90 decays are proportional to the amount of Y-90, so you start with almost nothing and approach an equilibrium within a few times the half life (~1 week).

If you start with a pure Sr-90 sample then its overall activity will double in that time frame, yes.