The addition of sulfite is, shall we say, a continuum.(posted 8327 days ago)As little as 10g/L will add up to 2/3 stop speed to fast films or to put it more accurately, restore speed lost from the film's nominal EI. I've found no significant effect on the speed of slow films.
More sulfite, up to around 50g/L, can provide a little less graininess but it's pretty subtle and again there'll be little effect on slow films.
If you already have established development times for Rodinal just use those as a starting point.
If you don't have a scale, a full plastic 35mm film cannister is approximately 50g of sodium sulfite.