Collars are a nice conservative strategy. They cap your upside but limit your downside, all for free! I would say to put a collar on when you want to own a stock but are worried about a future negative catalyst that could hammar the stock. Collars are better than puts at times because they are free or less expensive, and don't get devalued by time decay. I would not put a collar on an explosive stock, such as a volitle high beta stock like aig, because if the stock goes on a run you will miss out on a ton of gains.
You can put on a collar anytime you want, really it just depends on your preference. A lot of collars are hedges, not speculation. They are there to pretect gains. If you own a stock and it makes a huge run over several months, you may want to sell to protect gains but you don't want to pick a top. You think the stock may have further to go, but are unsure. Instead of buying a put to lock in gains, which is costly and has time decay, you can implement a cost free collar.
Investors use this option strategy a lot. It is really more of a stock play then an option play, because if the collar is cost-free then you don't have to worry about the greeks a whole lot.