Yeah I think you'll just have to try and see. I can get 7 gallons to a rolling boil on my gas stove, but lots of people have trouble. Keep it covered until it's boiling, and consider loosely wrapping the top part of the kettle with some heavy duty aluminum foil to keep the heat in there. I do all grain, so I start with hot wort and it brings it up to a boil in a little over an hour. If you're starting with cold water it'll take a lot longer.
But remember there's no shame in a partial boil, and most extract
recipe kits are designed for one.