My aquarium is pretty new (had it about 2 months) and my husband didn't know about cycling so 2 days after we got it set up I came home to a tiger oscar swimming around. We struggled a lot to keep the ammonia levels manageable. Even with a friend's seed material, it just didn't seem to take.
We ended up ordering Tetra SafeStart Plus (about $25 at Petsmart, $10 on Amazon), and that seemed to do the trick. After 6 weeks of going nowhere (ammonia levels of 2-4 ppm even with daily 10% water changes, 0 nitrites and 0 nitrates), the tank was cycled in 2 days. It's probably better to do it the "natural" way if you can, but if you've already got animals in the tank, and are desperate, it seems to work ok. It's been 2 weeks and the water conditions still seem very stable.