Sometime, Sometimes, and Some Time

All three of these adverbs are considered acceptable for formal writing.
Some time
When using some time, some is an adjective. It is describing the amount of time, and frequently implies a long period.

For some time, the world has been known to be round.

He has been studying Aramaic for some time.

It can also imply a fairly specific amount of time:

I require another couple of hours to finish this project today.

I require some time to finish this project today.

Sometime
When written as one word, this adverb implies a vague time in the future. It can frequently be replaced by someday.

The cure for cancer will be found sometime.

I’ll get around to finishing that book sometime later.

Give me a call sometime, and we’ll have coffee.

Sometimes
When written as one word and ending in s, this adverb implies occasionally.

Sometimes, I just don’t understand what that man is saying.

English grammar sometimes follows its own rules, and sometimes it doesn’t.