Travel time from Heathrow to Gatwick

Even if your flight arrives on time at Heathrow and there are short lines at immigration, it's impossible to make this connection. The two airports are about 50 miles away from each other. Sadly, even if your connecting flight was out of Heathrow you would probably still miss the flight:)

About how much time should I allow to make it between both airports--by train or bus?

National Express bus shows times of about 2 hours to get from Heathrow to Gatwick. And at that time of day, you'll be running into rush hour traffic, so it could be slower. But that's the travel time AFTER you clear immigration and get to the central bus station. There is no way you could make this connection.

You'd be cutting it potentially close if you STAYED at LHR. There is no way you make it. Call your travel agent if you haven't booked it yet and see if your destination from Gatwick is served out of Heathrow as well.

There is no train service between LHR and LGW. British Airways says that three hours is a "valid" connection time using National Express direct coaches and will book accordingly. But that's under absolutely ideal circumstances and presumes that both flights are on one ticket so the airline will have to put you on the next available flight if you miss the connection. If you have separate tickets, five hours is the minimum I'd recommend.

Thanks for everyone who responded. It saved me the purchase of a flight that most likely would not have worked!

This is a great example of why the helpline is so valuable. Real life experience.