For 20PPM production you can add the reducer AFTER the run BUT you have to keep the cell temp up over 75F because the solubility limit of silver oxide in water (ionic silver) at 75F is 21-22PPM. If you push the PPM (or at least try to) up over 20PPM you run a high risk of starting to precipitate out silver oxide which will NOT reduce and thats not good. Even still, once you add the reducer you need to heat the solution to over 120F so the reducer actually reduces. Reducers either don't work or if they do, you could measure their speed with a calendar at room temp.
For higher PPM production, you need to run hot (150F is fine as cfnisbet says) and add the reducer at the beginning of the run so that you're doing continuous production - the silver oxide dissolving in water (ionic silver) is being reduced faster than its being produced so you never coast up over the solubility limit.
The solubility limit for ionic silver at 150F is about 40PPM.
So long as you're doing continuous production, theoretically, you could push it as high as you want though I don't recall anyone here ever pushing it above 320PPM.
Something that may change the temp you want to run at for continuous production is the current you're running at. At higher currents, you'll be producing ionic silver faster and you may have to run hotter to speed up the reducer. I've never seen any real discussion of this here though so I can't speak to this.