How long (in milliseconds) is long enough to decide a user is actually hovering over an element?

I'm improving a widget that expands a dropdown list on hover.

One of the customers implementing the widget has requested a delay so we don't just slide up / down the drawer when a user is simply mousing round the page.

I'm fairly impatient as a person and testing with delays of ~ 1100ms are definitely too long waiting for something to happen.

But I'm not sure where's a sensible value sub 1100ms thats long enough to not be random mouse browsing but short enough to not have people get impatient.

I realise this is fairly specific but the widget is already embedded with zero delay on a large number of sites and I don't want to significantly reduce the UX across those sites by getting it wrong and realise I'm not a suitable test case.