Bag delays are not common, but within the realm of delays, they are almost all resolved within a week. A very few take longer and some, of course, are lost (stolen) and gone.

Your bag could be sitting somewhere at LHR, could have been loaded and sent to any place BA flies from LHR or could actually be sitting at PHL.

The system is "passive" meaning that nobody is physically out looking for your bag. Rather, your bag has been entered in BA's system and one it shares with other carriers. When a worker happens on a bag which isn't where it ought to be, they run the tag #, and your claim pops out and the bag gets routed to PHL and delivered to you.

Frustrating, but there is nothing you can do to make this happen any quicker.

If your baggage insurance requires you to make a claim within a time period of discovering it missing, make sure you do that in a timely manner.