Compound adjectives with a numeral and the name of a unit (as in “a one-hour walk” or “a three-storey building”) are written as solid if both constituents are single words, e.g. egyórás séta (“a one-hour walk”), from egy and órás). By contrast, if either element is a compound word (or both of them are), their compound will be written as separate words, e.g. tizenegy órás út (“a trip/flight of eleven hours”)), from tizen+egy + órás or egy hónapos utazás (“a journey of one month”), from egy + hó+napos. (See also the analogous rule for material designations.) In practice, this rule is sometimes disregarded, especially in casual texts and in case of less common compounds, as people often write them with a space (notwithstanding the single-word elements).
The -van/-ven (“-ty”) in round tens are suffixes, so e.g. negyvennapos (“forty‑day”) is written as solid. On the other hand, compounds with a number are always written separately, e.g. 6 éves. Also, adjectives (and nouns) for the denomination of coins, banknotes, and revenue stamps are written as solid, e.g. kétezerforintos bankjegy (“a 2000‑forint bill”), despite kétezerforintos having the compound numeral két+ezer (“2000”).