The sentence "when I have some free time" is correct and usable in written English. You can use it when you are talking about a future time when you will have the chance to do something, or when you are expressing your desire to do something. For example: "I'm hoping to finish my project soon, so that when I have some free time I can relax.".
To which Mr. Kagan, himself the father of three daughters, warmly replied: "Well, do it at this time of day, it's when I have some free time".
I'm the kind of person that when I have some free time, I'm on the golf course or out and about seeing people, so Twitter isn't something I really have too much interest in.
For me, the main way to do this is by batch cooking when I have some free time – usually on the weekends or in the evening when the kids are in bed.
I started consulting, but I had some free time.
"Whenever I had some free time, I'd go there and buy more".
When I have some precious free time, I find myself seeking out this kind of fare more and more.
Or I go play the guitar when I've got some free time.
Since I tried Ludwig back in 2017, I have been constantly using it in both editing and translation. Ever since, I suggest it to my translators at ProSciEditing.
Justyna Jupowicz-Kozak
CEO of Professional Science Editing for Scientists @ prosciediting.com