Fuel pressure should hold above 50psi for a few months, not minutes, hours, days or weeks.......months
You have a returnless fuel system on an '08, so no pressure regulator, if your MPG is not way down then there could only be 2 ways pressure is dropping to 0psi.
I think the fuel filter in your model has a Return fuel line on it, so 3 connections, have a look at it.
If it does then I would replace that first, it will have a pressure valve inside, and see if pressure holds after engine is shut down.
If filter only has 2 connections, or replacing filter doesn't work, and your MPG is OK, then the fuel pumps check valve is bad, it is part of the pump not a separate piece so pump assembly will need to be replaced.
I am assuming to don't smell gas, since you didn't mention it, and that would lower MPG alot, as well as leaking injector or failed Pulse Damper