any experience with Sterling background check??

That is fine...but how long do they take

Depends on how much stuff they have look through and if they need clarifications (I needed a reference from a past employer) and the whole thing took <1 wk

Nothing in particular.... they asked for 7 years of employment history. My employer 7 years ago was in a different country and so wondering about a situation where they are not able to reach them.

If you noticed, in the form they don't require the contact to be your boss. If they can't reach them, they will email you and ask for an alternate contact or copies of your paycheck. Pick any contact you want, just make sure they confirm the title and exact dates of employment.