I lot of times questions like these are to see how someone thinks about problems. What questions do they ask to clarify the problem. I'm not saying this is good question for that, but I wouldn't assume questions have a "right" answer.
Exactly, this is a pretty bad sign for a senior dev, to just blurb out a number. What is the connection used for? End users, a backoffice tool, or a batch process? then he should have asked: what is our SLA? What's in the DB, how big are the objects, the queries? How frequent are they?
Even so, I wouldn't venture any number if you don't do some profiling or at least some tests with the most common data, and with edge cases. It can be anywhere between milliseconds and full seconds, maybe more?.
Reminds me of an interview once, I asked: the website is slow, you're in charge of it, what do you do? He just said "buy more servers"...slow down there, Rockefeller.
