Instruction: Generate an answer to the question by utilizing the information contained in the document.

Input:
Question: How often does the prefilter need to be changed?
Document: Item received in good condition. However voltage was not compatible in Singapore. Need to get a transformer for it to function.
Luckily it's not too expensive. But still overprice for a dust filter.The main problem is now that I had to replace my pre filter. I realized it's a whole project where you need to cut the thing into the proper size. I'd like if i can get a filter that's actually made for my machine like the one that came with it perfectly pre-cut.
I'm not a fan of their BS marketing claims that these will reduce odors and gasses (with like a gram or two of charcoal in the prefilter!? )My next machine will be a real HEPA filter with like 7LBS of charcoal, THAT will do some odour and gas reduction!Something like AustinAir although they're not cheap, I think the filters are supposed to last like 3-5 years however!
bigger than expected, but well worth the investment. My daughter suffers from allergies and it has help allot. now she sleep comfortable.
Wonderful Allergen and dust remover. Two kitties kick up an amazing amount of dust and thisProduct takes care of it all.
Wish i had purchased a larger unit, however this works just as promised for my current needs. Would recommend this unit hand down. Thank you!!! !
Really works. Never had one before but after getting this one I bought 2 smaller ones for the bedrooms. Really like the air quality in the rooms now. Less sneezing too.
Other than the noise of wind, the air purifier also vibrates at med and high speed and generates a vibrating sound. The vibration goes away at turbo speed.New HEPA filter has a very strong smell. The smell is not desirable since the whole purpose of having a air purifier is to have a fresh air.
It works. No seriously, it really works. I was a major skeptic only because there are so many on the market. How can you really know you have the best or the right one. 5days later I'm amazed to see how much dirt it has picked up. Pleased!!! Highly recommended.Severe Allergy suffererAsthmaticPet ownerSeriously dust covered house.Relief
I've been carrying it between the bedroom and the living room for the time being. I can't say enough good things about this air purifier. My allergies are so minimal since I purchased it. It's really quite amazing. The purifier isn't so loud that I can sleep with it on the allergy setting. Turbo is a bit louder, but I don't even notice it when I'm awake. If you are an allergy sufferer like me and have been on the fence about trying one of these.
Answer: I've had my purifier a few months now. I've vacuumed the pre filter off several times, and it my opinion, it could be gently washed if necessary and will last a long time. I love this purifier!