Most often, sometime is one word: He will wash the car sometime.When some is used adjectivally with time to mean a short time, a long time, or an indefinite time, then it should be written as two words: She has not heard from her friend in some time.
Related words that can be discussed here include the pronoun anyone and the adverb anytime which are written as one word, while the pronoun no one is two words, though you will sometimes see noone. The two-word spellings of any time and any one are used in these constructions: At any time during the program, you may excuse yourself. / If you want any one of these scarves, just let me know. The intended meaning drives the choice but some rules can help you make a choice. Some time is the choice when a preposition comes before it or a helping word follows it: A short time ago, I finished the project. / Some time ago, we had lunch together. Some time can be replaced with an equivalent phrase (like a short time, a long time), but sometime cannot, e.g.: They will get tested sometime during the school year. Sometime means an indefinite or unspecified time, esp. at a time in the future. In speech, you will know to choose some time if your emphasis/stress is on time.