Sometime [adverb]: a not defined time, unspecified time
I’ll finish the great Australian novel sometime.
Sometimes [adverb]: occasionally, from time to time
Sometimes business owners think about going back to having a job.
Some time [phrase]: a period of time
For some time I have been planning to write another eBook.
This trio is based on the same two words merged into one, or not, and all relate to time so the differences are subtle enough it isn’t surprising some people misuse them.
Left as two words to be the phrase, ‘some time’ is the most precise and considered of the three – and it has more precision required to separate the two words so maybe that will help remember when to use the phrase rather than an adverb.