How helpful is indoor air filter for allergies?

Depends: If the air filter circulates the air in a room enough times it can help. You need a filter that will move enough air in the room to adequately filter it. If the filter is a hepa filter it can help. It needs to trap the small particles. If you have allergies to highly airborne allergens, it can help. A heavier allergen (dog dander) tends to drop to the floor where a filter cannot trap it.

May help: Agree that hepa filters are probably the best and most efficient as long as designed for the size of the room. Avoid devices that use ozone or ionizers. House dust mite is also heavy and will require other methods to decrease dust exposure.