I don’t know how to explain this, but it’s like……a hybrid of both options? Not as sharp or crisp as “hee hee.” More subdued. More sneaky. But not “heh heh” either, not so flat or low. Heeh heeh, if you will. But no pause in the middle. Heehheeh.
I tend to read it as 'heehee' by default-- it's the first thing I thought of when I saw the title of the thread. It's like a cutesy twin of 'teehee'. However, I do think it depends on the context of what's being said and who's saying it. It's not impossible for me to read 'hehe' as 'heh heh', it depends a lot on personality and intent. If 'hehe' is followed by a swish ~ or a cheerful emoji it's for sure 'heehee' coded in my mind. If there's a sheepish or mischevious tone or emoji tacked on I might read it as 'heh heh' instead.
"heh heh" for me. Otherwise I type it as "hee hee" if I want it to be read that way. However, judging by everyone's comments here, looks like I don't need to change the spelling lol