Is it timespan or time span?
Solution 1:
Merriam-Webster doesn't have an entry for timespan. Time span is correct, as you are referring to a span of time. I could see the argument for both being correct, though, as many other "Y of X" are converted into single word XY in english.
For programmers, timespan is used because spaces are not allowed in many places. If you are speaking to programers, timespan will be understood.
Solution 2:
NGram shows that time span is used far more.
