tldr; Levoit makes good products at a reasonable price, and their filters are good quality--which model are you considering? Levoit recommends 6-8 months on most of their products, I believe. I would also recommend considering the Coway 1512 Mighty and Winix 5500-2, both of which I believe are 12 month intervals and, if filter costs are a problem, have some good third parties that also offer filters for them (not certain about Levoit in this regard). The Coway filters, even the factory ones, are very inexpensive.

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in detail; Getting into the nitty gritty, it's hard to say exactly when a HEPA filter needs to be changed without a standalone air quality meter (and, to some extent, the ability to measure airflow through the unit itself). HEPA filters actually get more effective at capturing particulate matter the longer they are used. Then, after a point, their filtration efficiency falls because it gets clogged to a point where airflow through the filter becomes more difficult and decreases. This process gets expedited if extremely large particles cover the outside of the HEPA filter, so a lot of makers add reusable prefilters that capture hair, fur, and dust--clean these often and this can prolong the life of the HEPA filter (to a limited degree) and also the life of the unit (as the fan motor isn't working as hard).

The speed in which the above happens is highly dependent on the environment in which it is used and frequency/speed the unit is ran (Side note - I recommend running these units around the clock at the highest tolerable speed.) So, generally, they provide a rough filter interval estimate to try to account for average conditions. In a highly polluted environment, you may need to change it more often. In one with less pollution where you are constantly cleaning the prefilter, you could probably get away with longer change intervals (although some makers do require you to change the filter on their interval to maintain the factory warranty.)

Now, another thing to think about is carbon. Many filters today have carbon pellets impregnated into the HEPA filter, where a single filter contains both the HEPA + carbon components. The amount of carbon in most consumer units is very, very small, and its ability to impact VOCs for any prolonged period is extremely limited. This carbon will generally exhaust long before the HEPA portion of the filter, and some makers do appear to base their filter change intervals on this. With Levoit and most of the major manufacturers, I do think it is important to note how they are very much oriented towards the filtration of particulate matter. Their ability to reduce unwanted scents, and especially harmful VOCs (e.g., benzene, formaldehyde, etc.), is very, very limited. Arguably, particulate matter is going to be a bigger concern for most buyers, but if something is needed specifically for VOCs, a different kind of filter should be considered.