As a shiny hunter, I've found Dark Grass hunts in Gen 5 to be pretty enjoyable. However, I haven't done too many hunts in Gen 5 and the ones that I have done have been very decently below odds, so I'm going to do some math to calculate what a more average experience might look like.
If you're using Sweet Scent, I've found that it takes about 27 seconds to do an encounter and run away. Because the odds are also the average Pokémon required to get a shiny, it takes about 8192 Pokémon on average to get a shiny. Assuming that Dark Grass gives you a double encounter 50% of the time (I'm not sure if that's correct, but in my experience, I've never felt that the ratio was skewed, so 50/50 sounds about right to me), that means that it would take
8192 * 2/3 or 5461.3 encounters to get a shiny on average. This means that, in theory, it takes an average of 147,456 seconds, or 40.96 active shiny hunting hours to get a full odds shiny in Dark Grass.
If you're not using Sweet Scent, then I have no idea how long an average encounter would take, but I wouldn't think it would be any quicker than using Sweet Scent.
If you haven't shiny hunted very much, or haven't done any full odds hunts, then ~41 hours sounds like a lot, but finding a full odds shiny, especially one you've been hunting for, is one of the best experiences that the franchise has to offer, and Dark Grass helps a lot without diminishing that feeling (in my experience, anyway).
Edit: Also, keep in mind that this is an estimated average. The game could hate you more than these numbers suggest, or it could be nice and give you a shiny early. At the end of the day, it’s all just luck.