How much time?
 

Flying to AMS via LHR. Would like to meet a good friend now living in London. I can arrive at 8 am from JFK, depart to AMS at 5. Given customs, security lines, etc...how much time will I actually have?

1. Time through customs (or is there a transit shortcut)
2. Time to London
3. Time back to Heathrow
4. Time through security

Haven't seen her in a while and she is worth the trouble...

The T4 Heathrow Express terminal is back open. You should be able to make it to London in about an hour or just over an hour from arrival. That would leave plenty of time to have a nice long lunch.... Specially if she meets you in central london, so you do not have to travel a lot around. You may need to be back at Heathrow around 2 hours prior to your flight. After all, you will already have a boarding pass... so you just have to get through security (20 - 30 max).

The Heathrow Express station hasn't been closed.

It's the Tube station at T4 that was closed, but is now open again.

It only takes about 10-15 minutes extra to get somewhere in the Covent Garden area by Tube rather than by Heathrow Express plus onward journey, and is much cheaper.

I agree that 8 am to 5 pm is ample time to come into town and meet someone for lunch. But leave plenty of time for getting back to LHR. Security queues can be very long at the moment if you're unlucky.

If you get the Piccadilly Line Tube from T4 you go direct to South Kensington for various museums, Holborn where you can walk to the British Museum or Kings Cross for the British Library.

Whereas taking the HEX for anywhere except the immediate area around Paddington means getting a Tube / Bus / Taxi

If you change at Hammersmith or Earls Court BTW (walk across platform) you can easily reach Trafalgar Square / Westminster Abbey / House of Parliament / Big Ben / St Pauls / Tate Modern / Tower of London

AND it's also a LOT cheaper