I don't know of any websites with harvesting times or techniques, as harvesting times vary enormously with location, climate, weather, etc., and techniques are usually pretty self-explanatory with most plants. The key is to keep an eye on the seed capsules/pods and see what stage it's in. With mints (including salvias, thymes, rosemary, lavender, etc.), you just look down the capsule and see if the seeds have turned a dark colour. With plants with pods, you just keep an eye on whether they have started to turn brown, or whether the pods have started to split at the top. But generally, there are no absolutely general methods for harvesting them that I know of. If in doubt, wrap the seed head in plastic not long before you think they will be ready, and that should catch the seeds that come out. Watch out with impatiens, though, their seeds explode out (and to a somewhat lesser degree, violas too).