Spider mites are tiny. Usually found on undersides of leaves, they suck the juices from your plants. Yellow spots can show up on the plant- and because they are kin to spiders, ticks and other mites, they spread themselves around to other plants. If you AREN'T seeing them, that's a good thing. Neem oil and soap do work on controlling them- most other pesticides wont do such a good job. 2 weeks is the time it takes for eggs to hatch that were left behind by what you didn't catch the first time.