Time needed from arrival gate to Heathrow Express?

I know that an exact answer is impossible, but I'm trying to get a rough estimate. my girlfriend's flight on British Air from Los Angeles will arrive at noon at Terminal 5. I'm hoping that she could get to the Heathrow Express in time for a 13:12 departure, which would enable her to arrive at Paddington for a connection.
I know that, if she should miss that departure, there are frequent departures, and also other trains that she could take for her connection, but that schedule would be very convenient if it's possible.

There is an error in your query:-
my girlfriend's flight on British Air from Los Angeles will arrive at noon at Terminal 5.
should read:-
my girlfriend's flight on British Air from Los Angeles might arrive at noon at Terminal 5

Time to Heathrow Express will depend upon her entry qualifications - not specified and also whether she has baggage to reclaim - not specified. My guess is that she will miss the 13:12.

A bit touch and go. If the flight is on time and she makes an effort to be quick off the plane (i.e. chooses a seat nearish the front and walks quickly to overtake the dawdlers to immigration), then I would think she would have a good chance of making that train. Wouldn't advise you make your plans reliant on it though...

Thanks for the replies! You're right that her flight is only scheduled to arrive at noon. she has a US passport, and I think she'll have no checked luggage, and she can walk quickly, so maybe there's a chance?
If she misses that train, it's not a disaster, it's just that it would be very convenient, so that she can catch a connection at Paddington, and we'd meet about an hour later.

I've had occasions where I've alighted from the plane, gone through immigration, picked up my luggage and been on the Heathrow Express within twenty minutes of landing, and then I've had other times when it's been closer to two hours. At midday there will be more staff, so the queues should move fairly quickly. Without checked luggage that's one less 'wait', so it's quite feasible to make that connection at Paddington at 13.12. But .... have a Plan B in case it all goes pear-shaped ...

The only answer is you can never tell. There are so many variables on the day. The flight might be late, there may be lots of flights arriving at the same time, they may take a while to get passengers off, customs may pick on her, immigration may (and probably will) take a long time.....

I wouldn't think she could.

Customs may indeed pick on her if she's coming in from transatlantic with no checked in luggage. Bit odd that. Perfectly plausible of course, but they're entitled to ask.

Once she's through and cleared immigration, customs etc., it will take about 15-20 minutes to get on board the H.Express. As already mentioned it's the bit between the plane and the pre "Welcome to London" area that could prove to be a stumbling block.

Landing on time's a big if. That could vary 25 minutes either way, even if it takes off on time.

I'll bet a fiver she makes it easily.

That's a shaky fiver !