It depends on the airline, but most of the time its 25,000 miles for a free flight, but there are black out dates like Thanksgiving and Christmas. Personally I think the best use of them are for upgrading your coach seat to business class!
If it's through a card you can usually use them with any airline, but they have blackout dates and its sometimes hard to get the flight you want. If its through an airline directly you can fly with them or any of their miles partners. There are blackout dates but usually its easier to get flights, plus you can use the miles for access to their member lounges (waste of miles in my opinion) or to upgrade to first or business class.
Don't use airmiles unless it's really worth it. So, any short or cheap flight is not worth it. Just continue to save them up, and when you want to travel far or even out of the country, then use them.