A: "So, when's convenient for you?"
B: "Some time next week if possible."

"Some time" indicates an exact time or time range, for example, 3 o'clock in the afternoon, right?

If I replaced "Some time' with 'sometime', how does it sound to you?

'Sometime' is correct. 'Some time' = a considerable length of time.
Hi LiJ

Basically, the word "sometime" refers to an unspecified or not definitely known point in time. For example:

- I don't have time to correct your essay today or tomorrow. I'll have to do it sometime next week.

That means that I will do it next week, but precisely when is not specified or perhaps not yet known (i.e. maybe I'll do it on Monday morning, maybe on Thursday afternoon, maybe some other time next week).
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Thanks so much, Mr M.

So, if the speaker means a considerable length of time, 'Some time' is correct but the meaning is a little different to 'sometime', right? 
 Yankee's reply was promoted to an answer.
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Got the difference!
Thanks, Amy.