The general idea with developing film is that one has supplied enough light to the film for a nominal exposure, and in order to take the next step, a chemical reaction must occur to bring out that latent image.

What do you think happens if your developer were halfway to exhausted when you used it? You'd get only a partially developed piece of film.

So, we use a mix of developer that contains enough actual chemical for the batch to never reach exhaustion. If you were to leave your film in the developer for any length of time, you'd wind up with severely overdeveloped negs because there is enough developer chemical to keep on truckin'.

So, your controls become temperature (colder == slower reaction) and time (less == less development). Just about all the work for this is done for you by the manufacturer and posted on the massive dev chart, the film box, film spec sheets, etc.

Now, what does all that have to do with Rodinal? Simple, at most ratios, Rodinal is like a normal developer in that you have far more developer chemical than needed so you develop normally. This is any ratio up to 1+50.

At 1+100 or higher, you wind up with a measly 5mL of Rodinal inside of 500mL of liquid. This maxes out the volume on most people's smaller Patterson tanks and is so little actual chemical that it will reach exhaustion during development. When you get to this point, that becomes the goal! This technique is called stand development. Most begin with 5mL of Rodinal at 1+100 ratios and soup the film for an hour. Personally, I spin-agitate mine for a bit at 30min (semi-stand).

Rodinal displays different characteristics in it's development based on the ratio chosen1. There's little really to do except advise you to experiment. At anything 1+50 and below, you'll get usable negs following the standard process.

1+100 and up is for stand-development, should be considered highly experimental (don't risk your prized shots), and a word of caution about this: Rodinal loses speed at this level. If your plan is to stand-dev, then overexpose your shots 1/3 to 2/3 stop when taking them to make up for it. Again, though, this is experimental...YMMV.

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1: One could write a book on this so, to keep it brief: developer does not "work" at the same rate across the film. Highlight areas are dense and develop first, reaching a local exhaustion faster than shadow areas. This is why you agitate. Using higher ratios like 1+100 and not agitating so much will force a higher acutance, make the grains appear larger, and will have a compensating effect (lower contrast, help avoid blocking the highlights while the shadows develop). For this reason, most don't recommend Rodinal at all for high ISO films and especially not stand development.

Delta3200 @ ISO800 in 120 w/ 1+100 is about as grainy as I'll go. And it's very grainy. I like it for city/street imagery (comes off as gritty) but would avoid like the plague for portraits. At 135 film, the images to me are unusable no matter how well exposed/developed.

But, this is an art form. Be subjective and like what you like.