“Sometime” is an indefinite POINT in time; “some time” is an indefinite PERIOD of time. If you are thinking of “1:15” or “2:30,” it is one word; if you are thinking of “seven hours” or “ten minutes,” it is two words.
…Give me a call sometime/some time next week.
Are you thinking of giving him a call at 3:30 or for 20 minutes? This is a point in time; so it is one word.
…I will spend some time with him tomorrow.
Are you thinking of spending 3:15 or 20 minutes? This is a period of time; so it is two words.
There are many other “hints” that people use.
If you can take it out completely, it is usually one word.
…I will see you sometime tomorrow.
…I will see you tomorrow.
If you can take out “some,” it is usually two words.
…I have some time later today.
…I have time later today.
Then there are grammar requirements. If it is the subject of the sentence or the object of the preposition, it is two words because the sentence calls for a noun.
…Some time is needed to consider this.
…There will be some time later today.
…I have not seen him in some time.
…She has been here for some time.
It is two words in “some time ago” and “some time back” because it is a noun.
More to come.